Power Tips: How to measure frequency response in isolated supplies

Do you dread compensating the feedback loop of your isolated power supplies?  Where you choose to break the loop for your measurement will affect the difficulty of your task. 

In Power Tip 76, we discussed how to easily select the compensation components around a TL431 circuit.  While employing the equations in this Power Tip, it is crucial that you break the loop in a specific location.  There are two locations at which we could choose to break the loop. 

Most engineers are inclined to break the loop at the feedback resistor divider as shown in Figure 1.  After all, this is what we do with non-isolated bucks.  When we do this in isolated supplies, the inner loop becomes part of the plant and complicates the equations and the design procedure.  When we break the loop at the divider, we must:

  1. Inspect the stability of the open inner loop. 
  2. Then we must look at the closed loop response of this inner loop.  The closed inner loop is the plant, which is controlled by the outer loop.
  3. Ensure stability by selecting the compensation components around the TL431 in the outer loop.

Figure 1. Breaking the loop at the feedback divider complicates the measurement procedure.

By breaking the loop as shown in Figure 2, we can stabilize the power supply in one easy step.  Now the plant is defined as the transfer function from the output of the optocoupler to output of the power supply, and the two loops are included in our compensation rather than the plant.  This allows us to use the simple equations described in Power Tips: Compensating Isolated Power Supplies to quickly choose the compensation components around the TL431

Figure 2. Breaking the loop between the output and the entire TL431 circuit simplifies the measurement procedure.

Often, a 50 ohm resistor is incorporated into the circuit with the only purpose to provide a location for injecting a disturbance while measuring the loop.  When placed in the location denoted by Figure 2, the impedance of this resistor will affect the performance of the power supply.  The optocoupler current must flow through this resistor and will cause an error in regulation.  If you place a resistor in this location, a zero Ohm resistor must be used.  While performing a measurement of the loop, a 50 ohm resistor can be placed temporarily for injecting the disturbance.  Afterwards, the zero Ohm resistor must be replaced.


We have numerous isolated supplies in our Power Lab library of reference designs.  Here are a few examples which contain a zero Ohm place-holder resistor for measuring the feedback loop in the location shown by Figure 2:

  • PMP9203 – Universal AC Input 5V/2A USB Adaptor with Fixed Frequency Operation Reference Design
  • PMP9204 – Universal AC Input 5V/2A USB Adaptor with DCM/Valley-Switching and Opto Feedback Reference Design
  • PMP9720 – 48V-60Vdc Input, 12V/150W Active Clamp Forward - Reference Design

For more information on this topic please visit the EETimes article: Power Tips: Compensating Isolated Power Supplies