AN-31

C

7/04

11

U1

U2

D

S

C

OUTPUT

OUTPUT

RETURN

R6

L2

R12

R10

C10

R11

C14

R9

R4

C5

U3

TL431

C6

C11

C16

PI-2876-062204

CONTROL

CONTROL

DPA-Switch

Input

Return

+

Figure 9. Essential Components of the Feedback Circuit. The Schematic Does Not Show ESR of the Output Capacitors (Component

Designators are the Same as in the EP-21 Prototype).

The phase margin is specified at the crossover frequency. It

is the difference between the phase of the loop gain and 180

degrees. A stringent specification will call for a phase margin

of at least 60 degrees under worst case conditions. In no case

should the phase margin be less than 45 degrees.This means the

phase would have to decrease by that amount for the system to

become unstable. Phase margin is also related to the dynamic

characteristics of the system. A low phase margin suggests an

oscillatory response to a load step or other disturbance.

It is also important that the loop gain decrease in magnitude

beyond the crossover frequency. This requirement is generally

specified as gain margin. Gain margin is the difference between

0 dB and the magnitude of the loop gain at the frequency where

the phase is 180 degrees. An acceptable gain margin is greater

than10dB.Thismeansthemagnitudewouldhavetoincreaseby

thatamountforthesystemtobecomeunstable.Loopgainshould

be measured at worst case conditions (generally maximum

input voltage with maximum load) and at the extremes of the

specified ambient temperature, since important component

parameters (especially capacitor ESR) can change greatly with

temperature.

StabilizingahighfrequencyforwardDC-DCconverterpresents

some challenges due to the inherently high bandwidth of this

topology. Many DC-DC converter designs use cycle-by-cycle

current-mode control. The DPA-Switch uses classic voltage

mode control to allow operation at duty ratios greater than 50%

withouttheneedforthestabilizingramp(“slopecompensation”)

required with current-mode control. The fundamental system

characteristicsoftheforwardconverterincontinuousconduction

mode with voltage mode control call for a compensation

circuit with multiple poles and zeros to achieve the desired

loop response.

The crossover frequency for a control loop that uses

DPA-Switch

in a forward converter with an optocoupler should

be limited to 10 kHz or less at maximum input voltage and

room temperature. The DPA-Switch has one internal pole at

approximately 30 kHz to filter switching noise. Other poles at

higher frequencies contribute additional phase shift at 30 kHz.

The optocoupler has two poles at approximately 100 kHz. The

phase shift from these poles, combined with the phase shift

introduced by the LC filter at the output of the converter, is

difficult to compensate above 10 kHz.

The objective of the feedback design is to reduce the magnitude

of the loop gain to zero dB at a frequency of 10 kHz or less with

a phase margin near 60 degrees.Although system requirements

and the DPA-Switch fix some quantities that determine loop

characteristics, the designer can manipulate many components

in the feedback circuit to optimize loop stability. Figure 8

shows the essential components of a feedback circuit that uses

an ordinary TL431 regulator to achieve the high loop gain