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AN11130 Bias module for 50 V GaN demonstration boards Rev. 2 — 1 September 2015 Application note Document information Info Content Keywords GaN, bias Abstract This application note describes a bias controller module for GaN HEMT RF power transistors. It provides constant quiescent drain current with temperature, special bias and power sequencing, and overcurrent protection. AN11130 Bias module for 50 V GaN demonstration boards Revision history Rev Date Description v.2 20150901 Modifications v.1 20111208 • The format of this document has been redesigned to comply with the new identity guidelines of Ampleon. • Legal texts have been adapted to the new company name where appropriate. Initial version Contact information For more information, please visit: http://www.ampleon.com For sales office addresses, please visit: http://www.ampleon.com/sales AN11130#2 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 2 of 14 AN11130 Bias module for 50 V GaN demonstration boards 1. Introduction GaN HEMT RF power transistors require temperature-compensated gate bias voltages, similar to LDMOS devices, to maintain constant quiescent drain currents with temperature. They are depletion mode devices requiring special bias and power sequencing compared to LDMOS devices. This application note describes a bias controller module which provides these functions, and overcurrent protection. 2. Bias sequencing The most important consideration for DC with GaN HEMTs, is the bias sequencing. There are two issues to consider: • Never apply drain voltage when the gate is at 0 V, as the device draws excessive drain current. Thus, any GaN bias controller must include sequenced drain voltage switching. • For a given VGS, GaN HEMTs are likely to be potentially unstable at lower VDS. Therefore, decrease the gate voltage to below the pinch-off voltage VP (such as 3 V) while the drain voltage is being turned on and off. Figure 1 illustrates recommended power-up and power-down sequences for the drain and gate voltages. 60 discharge through power supply 40 20 0 VD VG 0 −2 −4 −6 0 10 20 0 10 20 30 40 50 time (ms) aaa-001666 (1) The typical discharge time shown is for illustration only, and is not intended as a recommendation. It is important that VGS is held at a voltage of less than VP until VDS is less than approximately 10 V. Fig 1. AN11130#2 Application note Gate and drain voltage sequence All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 3 of 14 AN11130 Bias module for 50 V GaN demonstration boards 3. Gate current Because the GaN HEMT gate terminal is a Schottky diode, bias generators must provide significant amounts of both positive and negative gate current: • GaN HEMTs have higher gate leakage currents than comparable LDMOS devices. The negative gate current can be as high as 500 A/mm of gate periphery at elevated junction temperatures; the gate current evaluates to 5 mA for a 100 W device operating at 200 C junction temperature. • When the device is driven into saturation, rectified positive gate current flows into the gate diode. At heavy RF compression, this gate current can exceed 1 mA/mm of gate periphery causing a possible gate current of 30 mA for a 100 W device. 4. Temperature compensation Similar to LDMOS devices, the gate threshold voltage for GaN devices is approximately proportional to temperature. The gate threshold voltage is the voltage required to maintain a constant quiescent drain current, which for Ampleon GaN devices, is about +1 mV/C junction temperature. However, practical temperature compensation circuits are obliged to monitor case temperature, where the temperature change is typically only half the junction temperature change. Thus, the bias controller must increase VGS by about +2 mV/C. 5. Summary The characteristics of the bias controller module described in the following section are summarized in Table 1. Table 1. AN11130#2 Application note Summary of bias controller characteristics Drain voltage 12 V to 80 V [1] Gate voltage 3 V to 1 V [1] Gate voltage adjustment range 700 mV typical Gate voltage temperature compensation +2 mV/C typical Gate current, negative 10 mA Gate current, positive 100 mA Gate voltage ripple 2 mV (p-p) Switched drain current 40 A [2] [1] Resistor values may have to be changed for part of range. [2] Dependent upon PCB layout. All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 4 of 14 AN11130 Bias module for 50 V GaN demonstration boards 6. Circuit description 6.1 Negative voltage generation The bias controller uses a switched-capacitor voltage inverter, U1, to generate a regulated 4 V from a single positive supply. The +5 V supply is generated from the high-voltage drain supply by linear regulator U4. U1 operates at a switching frequency of approximately 550 kHz; C3, R4, and C10 are used to reduce output ripple to less than 2 mV (p-p). When the output voltage is within 5 % of the 4 V set value, the REG output goes LOW. The REG output LOW signal acts as an active-LOW (power valid) signal to enable drain power to the GaN device. U4 IN +VD_IN C9 2.2 μF EN 8 U1 LT3010EMS8E−5 5 1 2 OUT +5V SENSE C7 1 μF VCC R9 1 SHDN −4V C3 10 μF 8 C10 10 μF LTC1261CS8−4 C1+ C4 100 nF R4 OUT 10 Ω 10 kΩ GND:4,9 6 C1− 2 3 GND:4 7 5 REG POWER_VALID_N COMP C6 100 pF aaa-001667 Fig 2. +5 V and 4 V voltage generation circuit 6.2 Drain voltage switching Most lower power HEMT bias controllers use a P-channel power MOSFET as the drain voltage load switch. This has the advantage of simplicity, often requiring nothing more than level-shifting transistors between the voltage inverter ‘power valid’ signal and the MOSFET gate. However, as load currents increase above 2 A to 5 A, the required MOSFET becomes large and expensive. Consequently, this bias controller uses a common hot-swap controller IC, U2 to drive an inexpensive N-channel MOSFET. AN11130#2 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 5 of 14 AN11130 Bias module for 50 V GaN demonstration boards Qdrain PSMN8R2−80YS Rsense +VD_IN 1 2 3 5 0.005 Ω 55 mV/Itrip +VD_OUT 4 D4 R17 68.1 kΩ BZX385−C11 U2 VCC UV ENABLE R3 10 kΩ 7 1 6 GATE LT4256−1CS8 C5 2 100 nF TIMER C11 100 nF SENSE 8 5 3 R22 10 Ω R21 75 kΩ R23 100 Ω R15 10 kΩ FB PGD POWER_GOOD GND:4 C1 10 nF aaa-001668 Fig 3. Drain voltage load switch circuit U2 contains an internal charge pump for driving the gate of the external N-channel MOSFET, Qdrain. It also includes a programmable ramp-up rate, and optional overcurrent fault detection and foldback current limiting. If the voltage drop across sense resistor Rsense exceeds 55 mV, U2 disconnects power from the drain, and leaves it latched off until power is cycled. Qdrain is a low-cost high-density TrenchMOS device in a small power SO8 package. It has a typical on-resistance of less than 10 m and a drain current limit of more than 80 A. 6.3 Temperature compensation The bias controller uses a small-signal PNP transistor (mounted in contact with the baseplate) to monitor temperature and generate a +8 mV/C compensating voltage. −4V R19 3.01 kΩ VTEMP R7 44.2 kΩ 3 1 Qtemp BC857B 2 R18 10 kΩ aaa-001669 Fig 4. AN11130#2 Application note Temperature compensating circuit All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 6 of 14 AN11130 Bias module for 50 V GaN demonstration boards −2.5 temperature compensation voltage (V) −2.6 −2.7 −2.8 −2.9 −3 −3.1 −3.2 −3.3 −3.4 −3.5 0 10 20 30 40 50 60 baseplate temperature (°C) Fig 5. 70 80 90 100 aaa-001670 Temperature compensating voltage 6.4 Gate voltage adjustment A variable voltage derived from the 4 V supply is summed with the temperature monitor voltage to generate a temperature-compensated gate voltage. R13 and R14 are selected to set the desired gate voltage trim range, and R6 is selected to provide the desired amount of temperature compensation. Figure 6 shows typical values for VGS = 1.6 V. U3 is a dual rail-to-rail high-current operational amplifier chosen because it is stable into any capacitive load. It can deliver more than 100 mA of output current to meet the positive gate current requirements of most GaN HEMTs. AN11130#2 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 7 of 14 AN11130 Bias module for 50 V GaN demonstration boards U3 LM7321MF 3 4 5 Rg 1 VGATE 10 Ω 2 R11 100 kΩ −4V C8 1 μF C9 1 μF R20 1 kΩ R19 3.01 kΩ R13 2 kΩ R6 30.1 kΩ 1 2 R5 1 kΩ R10 R7 44.2 kΩ 10 kΩ 3 3 1 C2 100 nF R14 2 kΩ R18 10 kΩ Qtemp BC857B 2 aaa-001671 Fig 6. Gate voltage adjustment circuit −1 VG pot fully CW −1.1 pot mid-scale pot fully CCW −1.2 −1.3 −1.4 −1.5 −1.6 −1.7 −1.8 −1.9 −2 0 10 20 30 40 50 60 baseplate temperature (°C) Fig 7. 70 80 90 100 aaa-001672 Gate voltage The output impedance of the bias source is low (less than 1 ) because of the feedback around operational amplifier U3. However, HEMT applications usually require a series gate resistor of 5 to 20 , instead of a bias inductor, to ensure low-frequency device stability. Because the GaN HEMT quiescent gate current can increase at high temperatures, a significant voltage drop can be developed across this gate resistor, increasing VGS by 200 mV or more. This can be a serious problem because the increased gate voltage can push the HEMT operating point into a region of instability or even cause thermal runaway. AN11130#2 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 8 of 14 AN11130 Bias module for 50 V GaN demonstration boards One solution is to provide DC feedback from the HEMT gate terminal to the operational amplifier inverting input, allowing it to compensate for voltage dropped across gate resistor Rg. Feedback through R20 compensates for voltage dropped across Rg. To minimize RF non-linearity and memory effects, make sure that 1 / (R20 C8) is less than the lowest modulation frequency of the RF signal. 6.5 Resistor value selection The resistor values in the schematic are for a 50 V device with a nominal VGS of 2.1 V, +2 mV/C VGS temperature coefficient, and 11 A IDS overcurrent sense. Other nominal operating conditions may require resistor value changes: • R17 determines the supply voltage at which the drain is connected. R17 = R3 (VD / 4 1), so VD = 31 V with the 68.1 k value in the design. • R21 determines the supply voltage at which operating VGS is applied. R21 = R15 (VD / 4.45 1), so VD(good) = 38 V with the 75.0 k value in the design. • Rsense determines the foldback current limit. ILIMIT = 0.055 / Rsense, so ILIMIT = 11 A with the 5 m value in the design. 6.6 Full schematic The complete schematic of the bias controller combines the preceding subcircuits with ‘glue circuitry’ and several (possibly optional) additional convenience features. • LEDs D1 and D2 provide visual indication of valid gate and drain voltages. • Switch S1 allows the GaN amplifier to be placed easily in ‘standby’, where the nominal gate voltages are applied, but drain power is disconnected. • When S1 is set to ‘Vd on’ and the drain supply is not fully on (ramping up or down), diode D3 and MOSFET Q3 turn on MOSFET Q2, pulling the gate voltage down to 4 V. Table 2. Bias module bill of materials Component Description Value C9, C12 capacitor, 100 V 10 % X7R, 1206 1 F C7, C8 capacitor, 50 V 10 % X7R, 0805 1 F C2, C4, C5, C11 capacitor, 50 V 10 % X7R, 0805 100 nF C6 capacitor, 50 V 5 % NP0, 0805 100 pF Remarks C3, C10 capacitor, 10 V 20 % X7R, 0805 10 F C1 capacitor, 50 V 10 % X7R, 0805 10 nF D1 LED, green, 0805 D2 LED, yellow, 0805 D4 diode, Zener 11 V 300 mW BZX385-C11 D3 dual diode, common-cathode BAV74 L1 ferrite bead, 200 mA, 0805 Q1, Q2, Q4 transistor, N-ch MOS 30 V 0.8 A Q3 transistor, P-ch MOS 30 V 0.8 A R13, R14 resistor, 1 % 100 ppm CF, 0805 2.00 k R5 potentiometer, 5t cermet 1 k AN11130#2 Application note 1000 mH? BSH103 BSS84 All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 9 of 14 AN11130 Bias module for 50 V GaN demonstration boards Table 2. Bias module bill of materials Component Description Value R19 resistor, 1 % 100 ppm CF, 0805 3.01 k R7 resistor, 1 % 100 ppm CF, 0805 44.2 k R3, R9, R10, R12, R15, R16, R18 resistor, 1 % 100 ppm CF, 0805 10.0 k R6 resistor, 1 % 100 ppm CF, 0805 30.1 k R4, R22, R24 resistor, 1 % 100 ppm CF, 0805 10.0 k R1, R8, R20 resistor, 1 % 100 ppm CF, 0805 1.00 k R17 resistor, 1 % 100 ppm CF, 0805 68.1 k R23 resistor, 1 % 100 ppm CF, 0805 100 k R2, R11 resistor, 1 % 100 ppm CF, 0805 100 k R21 resistor, 1 % 100 ppm CF, 0805 75.0 k S1 switch, SPDT right-angle SMD Tyco 1437575-1 U4 voltage regulator, 5 V 100 mA Linear LT3010EMS8E-5 U1 switching capacitor inverter Linear LTC1261CS8-4 U2 hot swap controller Linear LT4256-1CS8 U3 rail-rail opamp National LM7321MF AN11130#2 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 Remarks © Ampleon The Netherlands B.V. 2015. All rights reserved. 10 of 14 xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxx x x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xx xx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx x x xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxx AN11130#2 Application note 6 5 R24 4 10 Ω 3 D4 L1 BLM21BD102 U4 LT3010EMS8E−5 IN 8 C9 1 μF C12 1 μF EN 5 S1 1437575−1 standby U1 LTC1261CS8−4 1 OUT 2 SENSE +5V C7 1 μF R9 SHDN 3 6 OUT VCC 1 8 3 10 kΩ GND:4,9 C4 100 nF C1− 7 2 5 3 REG 2 C6 100 pF GND:4 UV R12 1 R8 3 R2 100 kΩ BSS84 Q3 3 1 kΩ 2 BAW56 2 3 1 D3 5 C11 100N GND:4 10 kΩ D3 1 2 7 SENSE R22 10 Ω R21 75 kΩ R23 100 Ω R15 10 kΩ 6 GATE 2 TIMER R1 1 kΩ S npn mosfet 1 LT4256-1CS8 VCC 8 C5 100 nF R3 10 kΩ D1 HSMG−C170 green = VG on COMP R17 68.1 kΩ 2 Q1 BSH103 1 G switching 3 FB PGD C1 10 nF Q4 BSH103 D2 HSMY−C170 yellow = VD on 1 BAW56 3 U3 LM7321MF 3 1 2 Q2 BSH103 3 5 4 2 1 C8 1 μF R11 100 kΩ R16 10 kΩ 10 R20 11 1 kΩ 12 -4V R13 2 kΩ VG FB C10 10 μF R19 3.01 kΩ R6 1 2 R10 30.1 kΩ 10 kΩ R7 44.2 kΩ R14 2 kΩ 7 8 3 C2 100 nF R18 10 kΩ 9 C temperature B compensation E npn transistor aaa-001673 Fig 8. Bias module schematic AN11130 11 of 14 © Ampleon The Netherlands B.V. 2015. All rights reserved. R5 1 kΩ Bias module for 50 V GaN demonstration boards Rev. 2 — 1 September 2015 All information provided in this document is subject to legal disclaimers. C1+ BZX384-C11 D drain U2 1 VD on C3 10 μF R4 10 Ω 2 V+ AN11130 Bias module for 50 V GaN demonstration boards 7. PCB layout aaa-001674 Fig 9. PCB top-side layout aaa-001675 Fig 10. PCB bottom-side layout 8. Abbreviations Table 3. AN11130#2 Application note Abbreviations Acronym Description LDMOS Laterally Diffused Metal Oxide Semiconductor MOSFET Metal Oxide Semiconductor Field Effect Transistor All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 12 of 14 AN11130 Bias module for 50 V GaN demonstration boards 9. Legal information 9.1 Definitions Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. Ampleon does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information. 9.2 Ampleon does not accept any liability related to any default, damage, costs or problem which is based on any weakness or default in the customer’s applications or products, or the application or use by customer’s third party customer(s). Customer is responsible for doing all necessary testing for the customer’s applications and products using Ampleon products in order to avoid a default of the applications and the products or of the application or use by customer’s third party customer(s). Ampleon does not accept any liability in this respect. Export control — This document as well as the item(s) described herein may be subject to export control regulations. Export might require a prior authorization from competent authorities. Disclaimers Limited warranty and liability — Information in this document is believed to be accurate and reliable. However, Ampleon does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. Ampleon takes no responsibility for the content in this document if provided by an information source outside of Ampleon. Evaluation products — This product is provided on an “as is” and “with all faults” basis for evaluation purposes only. Ampleon, its affiliates and their suppliers expressly disclaim all warranties, whether express, implied or statutory, including but not limited to the implied warranties of non-infringement, merchantability and fitness for a particular purpose. The entire risk as to the quality, or arising out of the use or performance, of this product remains with customer. In no event shall Ampleon be liable for any indirect, incidental, punitive, special or consequential damages (including - without limitation - lost profits, lost savings, business interruption, costs related to the removal or replacement of any products or rework charges) whether or not such damages are based on tort (including negligence), warranty, breach of contract or any other legal theory. In no event shall Ampleon, its affiliates or their suppliers be liable to customer for any special, indirect, consequential, punitive or incidental damages (including without limitation damages for loss of business, business interruption, loss of use, loss of data or information, and the like) arising out the use of or inability to use the product, whether or not based on tort (including negligence), strict liability, breach of contract, breach of warranty or any other theory, even if advised of the possibility of such damages. Notwithstanding any damages that customer might incur for any reason whatsoever, Ampleon’ aggregate and cumulative liability towards customer for the products described herein shall be limited in accordance with the Terms and conditions of commercial sale of Ampleon. Right to make changes — Ampleon reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. This document supersedes and replaces all information supplied prior to the publication hereof. Suitability for use — Ampleon products are not designed, authorized or warranted to be suitable for use in life support, life-critical or safety-critical systems or equipment, nor in applications where failure or malfunction of an Ampleon product can reasonably be expected to result in personal injury, death or severe property or environmental damage. Ampleon and its suppliers accept no liability for inclusion and/or use of Ampleon products in such equipment or applications and therefore such inclusion and/or use is at the customer’s own risk. Applications — Applications that are described herein for any of these products are for illustrative purposes only. Ampleon makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification. Customers are responsible for the design and operation of their applications and products using Ampleon products, and Ampleon accepts no liability for any assistance with applications or customer product design. It is customer’s sole responsibility to determine whether the Ampleon product is suitable and fit for the customer’s applications and products planned, as well as for the planned application and use of customer’s third party customer(s). Customers should provide appropriate design and operating safeguards to minimize the risks associated with their applications and products. AN11130#2 Application note Notwithstanding any damages that customer might incur for any reason whatsoever (including without limitation, all damages referenced above and all direct or general damages), the entire liability of Ampleon, its affiliates and their suppliers and customer’s exclusive remedy for all of the foregoing shall be limited to actual damages incurred by customer based on reasonable reliance up to the greater of the amount actually paid by customer for the product or five dollars (US$5.00). The foregoing limitations, exclusions and disclaimers shall apply to the maximum extent permitted by applicable law, even if any remedy fails of its essential purpose. 9.3 Trademarks Notice: All referenced brands, product names, service names and trademarks are the property of their respective owners. Any reference or use of any ‘NXP’ trademark in this document or in or on the surface of Ampleon products does not result in any claim, liability or entitlement vis-à-vis the owner of this trademark. Ampleon is no longer part of the NXP group of companies and any reference to or use of the ‘NXP’ trademarks will be replaced by reference to or use of Ampleon’s own Any reference or use of any ‘NXP’ trademark in this document or in or on the surface of Ampleon products does not result in any claim, liability or entitlement vis-à-vis the owner of this trademark. Ampleon is no longer part of the NXP group of companies and any reference to or use of the ‘NXP’ trademarks will be replaced by reference to or use of Ampleon’s own trademarks. All information provided in this document is subject to legal disclaimers. Rev. 2 — 1 September 2015 © Ampleon The Netherlands B.V. 2015. All rights reserved. 13 of 14 AN11130 Bias module for 50 V GaN demonstration boards 10. Contents 1 2 3 4 5 6 6.1 6.2 6.3 6.4 6.5 6.6 7 8 9 9.1 9.2 9.3 10 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Bias sequencing . . . . . . . . . . . . . . . . . . . . . . . . 3 Gate current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Temperature compensation . . . . . . . . . . . . . . . 4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Circuit description . . . . . . . . . . . . . . . . . . . . . . . 5 Negative voltage generation . . . . . . . . . . . . . . . 5 Drain voltage switching. . . . . . . . . . . . . . . . . . . 5 Temperature compensation . . . . . . . . . . . . . . . 6 Gate voltage adjustment. . . . . . . . . . . . . . . . . . 7 Resistor value selection . . . . . . . . . . . . . . . . . . 9 Full schematic. . . . . . . . . . . . . . . . . . . . . . . . . . 9 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Legal information. . . . . . . . . . . . . . . . . . . . . . . 13 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Please be aware that important notices concerning this document and the product(s) described herein, have been included in section ‘Legal information’. © Ampleon The Netherlands B.V. 2015. All rights reserved. For more information, please visit: http://www.ampleon.com For sales office addresses, please visit: http://www.ampleon.com/sales Date of release: 1 September 2015 Document identifier: AN11130#2