Download Unit 7: MOSFET-Output Motor Controller

5/1/17
Unit 7: MOSFET-Output Motor Controller
Page 1 of 4
Introduction: Coreless DC motors can have extraordinarily low armature inductance. For
example, the Faulhaber 2657W012 motors used in the Cornell ranger robot have an inductance
of about 90 uH, and a terminal resistance of about 1.4 ohms. Efficient PWM motor control
requires that the time constant of the RL circuit formed by the motor and H-bridge output stages
be much longer than the period of the PWM output drive. If it isn’t, the I2R loss in the armature
during torque generation can be as high as twice what would be expected during DC operation at
the same torque, due to ripple in the current. Even assuming zero output stage resistance, the LR
time constant for the motor, given by L/R, is only 64 uS. A general rule is to have a PWM period
five times lower, to give negligible ripple, resulting in a required PWM frequency of at least 70 to
80 kHz. Interestingly, the Faulhaber motor controller for this motor uses 70 kHz. Note that
increased circuit resistance increases the required frequency, so any output driver resistance or
wire resistance would lead to even higher PWM frequency requirements.
Thus, our previous designs using the ST VNH2 and the Infineon BTN7971, developed for
automotive power windows, are not suitable for coreless motors; their maximum rated PWM
frequency is 20 to 25 kHz, and while there is some reason to think they could be operated at up
to twice that, 70 kHz is probably too high.
Module design goals: This module is intended to provide a tiny motor controller capable of:
- Driving brushed DC motors
- Allowing battery input voltages of up to 24 volts
- Up to 100 kHz PWM operation
- Continuous output current of up to 10 amps
- On a miniature 30 x 45 mm board with no special heat sinks
- Simple control from a microcontroller, with no special timing requirements.
- Versatile control options, allowing full four-quadrant control, regeneration, efficient synchronous
switch operation, free-run, and braking.
- At least partially isolate the motor power circuits from the microcontroller and network circuits.
However, the assumption is that all circuits ultimately operate off the same power supply
(battery), and share a common ground at one central location. Thus, isolation voltage capability is
on the order of the battery voltage – about 50 volts, not the thousands of volts of which the
isolation circuits themselves are capable.
In addition, it is designed to have a number of safety and monitoring capabilities, including:
- Battery voltage measurements
- Battery current measurements
- Motor current measurements
- Isolation of digital control inputs
- Analog measurements, though not galvanically isolated, are through high-value (~100K) series
resistors.
- Temperature sensing and automatic overtemperature shutdown.
Created 11/18/08 by Jason Cortell
Cornell University
Modified 5/1/2017 2:17:00 AM by Jason Cortell
840962246
5/1/17
Unit 7: MOSFET-Output Motor Controller
Page 2 of 4
Design considerations and choices:
MOSFETS: These are the heart of any motor controller. Toshiba devices in their custom SOPA-8
package were selected. These have the board space requirements of an SOP-8 device, but with
a central thermal pad soldered to the board to allow greater heat dissipation. In addition, they are
fast, have low gate drive requirements, and very low on resistance. Other manufacturers,
including Fairchild, Vishay Siliconix, and International Rectifier, make devices in similar packages
which could be substituted either directly or with very minor board changes.
Summary of specs for the TPCA8014, 8015, and 8016:
Parameter
Drain-Source breakdown voltage
Maximum drain current
Drain-source on resistance, Vgs = 10V (typ.)
Gate resistance
Total gate charge, Vgs = 10V
Input capacitance
Reverse transfer capacitance
Output capacitance
Rise time
Fall time
TPCA8014
40V
90A
7.1 mOhm
1.0 Ohm
22 nC
1365 pF
110 pF
480 pF
5 nS
4 nS
TPCA8015
40V
105A
4.4 mOhm
1.4 Ohm
37 nC
2155 pF
200 pF
780 pF
5 nS
10 nS
TPCA8016
60V
75A
16 mOhm
1.0
22 nC
1375 pF
70 pF
340 pF
4 nS
3 nS
The TPCA8014 was chosen for the initial design, since the lower gate capacitance reduces the
drive power requirements for a given PWM frequency, and allows the gate to be charged and
discharged more rapidly with a given driver circuit, thus reducing switching losses. In addition,
the low reverse transfer capacitance helps prevent the MOSFET from turning on when subjected
to high rates of voltage increase (dV/dt) at its drain terminal. This is typically a problem for a lowside switch when its corresponding high-side switch turns on, but in some motor controller areas
of operation this can also happen to the high-side switch. Thus, for these devices, the turn-on
time needs to be slowed down to prevent accidentally turning on the other MOSFET in each halfbridge. In this circuit, a gate resistor is used, with a Schottky diode installed to allow more rapid
gate discharge during turn-off.
The TPCA8015 would be a better choice for lower frequencies (possibly including 100 kHz) and
higher currents, where the lower gate resistance losses would more than outweigh the higher
switching losses.
The TPCA8016 would allow operation at higher voltages (up to 40 or 50 volts?), but with lower
currents due to the higher “on” resistance.
Gate driver circuit: Standard logic circuits don’t work well to drive high-speed power MOSFETs,
because the available instantaneous current for gate charging and discharging is not high
enough. In addition, if an N-channel MOSFET is used for the high-side switch, the gate drive
voltage must float along with the source terminal of the device, reaching voltages above the
supply when the device is on.
A gate driver for this circuit needs to have medium-high voltage capability (at least 60 volts),
generate the required high-side gate drive voltage internally, have high output current drive
capability, high switching speed, automatic dead-time generation, allow independent operation of
the high- and low-side switches (for versatile operation), small size, and use little power itself.
Dead-time generation is important to eliminate shoot-through, in which both output MOSFETS
turn on at the same time and short-circuit the power supply – bad, even if usually brief. Although
Created 11/18/08 by Jason Cortell
Cornell University
Modified 5/1/2017 2:17:00 AM by Jason Cortell
840962246
5/1/17
Unit 7: MOSFET-Output Motor Controller
Page 3 of 4
this can be done by the microcontroller, it would be preferable to have it done in hardware to
simplify the microcontroller software and interface.
The device chosen was the Micrel MIC4102, operating at up to 100 volts with adaptive anti-shootthrough protection, and capable of 2 A peak source current and 3 A peak sink current. Normal
operation uses the PWM input, and one or the other switches will be on depending upon whether
the PWM is high or low. In addition, it has a low-side enable input, to allow the low-side switch to
be shut off even when the high-side switch is off. The closest potential substitute found was the
National LM5104, which lacked the low-side enable input and had lower output drive capability,
but had additional dead-time adjustments and an exposed center pad for heat dissipation.
Connector pinouts:
Virtual module interface connector (layer 2 of printed circuit board)
Signal name
7_5V5
Signal type**
Power input
Position*
1.125
7_CHA_1
Digital output
1.100
7_BAT_IOUT
Analog output
1.050
7_5V5
Power input
1.025
7_DOUT_1
Digital output
1.000
7_CHB_1
7_ENC_V_1
7_DCLK_1
Digital output
Power input
Digital input
0.950
0.925
0.900
7_LSA_IN
Digital input
0.600
7_PWMA_IN
7_MIOUT
Digital input
Analog output
0.575
0.550
7_BAT_VOUT
Analog output
0.500
7_WDT_IN
Digital input
0.450
7_3V3
Power input
0.375
7_!OS
Digital output
0.350
7_LSB_IN
Digital input
0.325
7_VTEMP
Analog output
0.300
7_PWMB_IN
Digital input
0.275
Created 11/18/08 by Jason Cortell
Cornell University
Comments
5.5 volt power to 5 volt LDO regulator for
analog circuits (< 10 mA or so).
Quadrature channel A from encoder 1;
voltage levels, etc. depend on encoder
V proportional to battery current, 0 to 5V
range, 2.5 V zero reading.
5.5 volt power to 5 volt LDO regulator for
analog circuits (< 10 mA or so).
Data from absolute encoder serial
synchronous interface 1
Quadrature channel B from encoder 1
V+ power to encoder 1
Serial synchronous encoder 1 interface
clock input
3.3 volt logic low-side A enable input to
digital isolator
3.3 volt logic PWM A input to isolator
V proportional to average motor current, 0
to 5V range, 2.5 V zero reading. AD8206
has 100 KHz bandwidth
V proportional to battery voltage, 0 to 5V
range, 2.5 V zero reading.
3.3 volt logic input to watch-dog timer. Apply
a software-generated frequency of at least
200 Hz to enable MOSFET drivers.
3.3 volt logic power supply input (<5 mA or
so)
3.3 volt logic, thermal overload active low
(goes low when the temperature is too
high).
3.3 volt logic low-side B enable input to
digital isolator
V proportional to board temperature near
half-bridge B. 0 to 3.3 volt range from LM26.
Scale is -10.82 mV/degree C.
3.3 volt logic PWM A input to isolator
Modified 5/1/2017 2:17:00 AM by Jason Cortell
840962246
5/1/17
Unit 7: MOSFET-Output Motor Controller
7_CHB_2
7_CHA_2
7_DCLK_2
Digital output
Digital output
Digital input
0.250
0.225
0.200
7_ENC_V_2
7_DOUT_2
Power input
Digital output
0.150
0.050
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Quadrature channel B from encoder 2
Quadrature channel A from encoder 2
Serial synchronous encoder 1 interface
clock input
V+ power to encoder 2
Data from absolute encoder serial
synchronous interface 2
Layer 15 (third down)
Digital ground
0.020 –
plane/polygon,
0.765
layer 15
7_AGND
Analog ground
0.775 –
Layer 15 (third down)
plane/polygon,
1.160
layer 15
* In inches from bottom board edge.
** Input refers to signals going to the board; outputs come from the board
7_DGND
Bugs to fix in new versions:
1) Use thermals to enable easier replacement of MicroMaTch connectors. The plastic housing
can be lifted off, enabling the pins to be removed individually, but the ground and power pins are
hard to get hot enough.
Created 11/18/08 by Jason Cortell
Cornell University
Modified 5/1/2017 2:17:00 AM by Jason Cortell
840962246