Download Subject: Switching Regulators Topic: DC to DC Convertors The most

Subject: Switching Regulators
Topic: DC to DC Convertors
The most efficient of all DC to DC convertors is the switching regulator. Switching regulators are
significantly more power efficient than linear regulators, of course, at the expense of having
more output noise generated through the switching process. But switching regular topologies
allow for a wide range of applications including step-up (boost), step-down (buck), and
inverting voltage regulation (boost/buck).
Embedded within a switching regular is the on/off power switch (often times a Vertical Metal
Oxide Semiconductor VMOS but it can also be a bi-polar device as well). The on/off duty-cycle
of the power switch sets how much energy will be stored-up, and then, be transferred to the
load. Whereas a linear regulator uses a power inefficient voltage drop across a resistor to
regulate the voltage, relatively speaking, a switching regulator dissipates almost no power! The
secret lies in the power switch. When the switch is open, the voltage is high across the switch
but the current is zero. When the switch is closed, the current is high through the switch but
the voltage is zero! Because the voltage and current are 90 degrees out of phase through the
inductor (that also does not have a DC voltage drop) the switching regulator can approach
extremely high percentages in terms of power efficiency.
Fig.1 Step-up switching regulator (Boost Convertor)
Fig.2 Step-down regulator (Buck Convertor)
Let’s briefly explain a step-up switching regulator function in regards to a boost convertor (see
Fig.1). A boost convertor has the ability to output a larger voltage than is applied to the input.
The simplified boost convertor in Fig.1 contains an inductor, the power switch, a rectifying
diode, and a capacitor. The inductor’s main function is to store energy and to limit the current
slew rate through the power switch (otherwise a high peak current would be limited by the
switch resistance alone). At a steady-state condition, with the switch open, the inductor
charges the capacitor until +Vout is equal to +Vin (and the diode current goes to zero). When
the switch closes, the input voltage +Vin is applied across the inductor as the diode prevents
the capacitor from discharging +Vout (which is still equal to +Vin) to ground.
The current through the inductor rises linearly, di/dt, (with respect to the time the switch is
closed) at the rate of +Vin/L. Of course when the switch opens again, the inductor current then
flows through the rectifying diode to further charge the capacitor with the voltage increasing at
dv/dt (with respect to the time the switch is open) at I/C. If the power switches duty cycle
(D=tclosed/(tclosed+topen)) equals 50%, than +Vout will ideally be Vin+Vin or twice the applied
input voltage (because the average inductor voltage in steady-state must equal to zero)! Of
course the duty cycle D can be varied accordingly and thereby adjusting the output voltage with
the equation of Vout=Vin/(1-D). This gives the user great flexibility in using the boost convertor
topology in trading off the DC input voltage (+Vin) versus the required multiplied DC output
voltage (+Vout) that would be required to drive the circuit load within a given overall power
efficiency.
Of course, it is given that the ideal boost or buck convertor circuit provides tremendous
advantages in power efficiency. The largest power-loss factor in a boost or buck convertor is the
rectifying diode. The power dissipated (in heat) is simply the forward voltage drop multiplied by
the current going through it. To maximize efficiency, another power switch can replace the
diode. This rectifier switch would then be open when the main switch is closed in a breakbefore-make type of clock scheme to prevent both switches being on simultaneously. In this
type of configuration, power efficiencies of well above 93% can be achieved.
Figure 3 PT1203 2A, 18V, 350 KHz synchronous switching regulator from Powtech
Now let’s look at a step-down convertor (buck) such as the PT1203 2A, 18V, 350 KHz
synchronous switching regulator from Powtech (www.crpowtech.com) shown above in Figure
3. The PT1203 is a pulse-width-modulated (PWM) synchronous step-down switch mode
regulator with two internal power MOSFET’S. It achieves 2A continuous output current over a
wide input supply range with excellent load and line regulation. Current mode operation
provides fast transient response and eases loop stabilization. It also has fault condition
protection which includes cycle-by-cycle current limit and thermal shut-down. The PT1203
requires a minimum number of external components external to either a SOP-8 or ESOP-8
package (see Figure 4 below).
Figure 4 PT1203 Application Circuit with external components
This switching regulator uses a MOSFET transistor as the power switching device. It has an on
board oscillator which sets the switching frequency with a single external capacitor for 350 kHz
(typical operation). The output can switch up to 2A with current limit and thermal shutdown
capability. Overall, switching regulators are best when power efficiency is critical (such as in a
portable battery powered device) and when the +Vin power supply is usually a DC voltage and a
higher +Vout output voltage is required. Also, at high levels of power (above a few watts),
switching regulators are cheaper—due to the fact they generate much less heat and therefore
the cost and space due to complex thermal design is eliminated. Keep a close eye on the output
voltage ripple of the switching regulator, and the effect that it will have on the circuit that it
must drive, and the design will be greatly enhanced.
When designing in a switching regulator, it is important to simply break down the system into
the required functions and performance specifications that make up the system and address
each performance limiting factor. Depending on the overall system specification, these
numbers will determine many of the required analog performance specifications of the system.
The number one thing is to remember that every node in a circuit has some type of component
connected to it and it is also both an input and an output in some way. Understanding the
positive and adverse effects of this single concept will greatly enhance your ability to design the
system.
Kai ge from CADEKA
(www.cadeka.com)