Power Tips: How to Measure Frequency Response in Isolated Power Supplies

When choosing the compensation components around the TL431 circuit, it is critical to break the loop at a specific location. We can choose to break the loop at two locations.

Most engineers prefer to break the loop at the feedback resistor divider shown in Figure 1. After all, that’s what we do in the non-isolated buck circuit. When we do the same in this isolated power supply, the inner loop becomes part of the power plant equipment and complicates the equations and design process. When we break the loop at the divider, we must:

Check the stability of the internal open loop.

We must then look at the closed loop response of this inner loop. The closed inner loop is the power plant equipment that is controlled by the outer loop.

Stability is ensured by selecting the compensation components around the TL431 within the external loop.

Figure 1. Breaking the loop at the feedback divider location complicates the measurement process.

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

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

Often, a 50 ohm resistor is included in the circuit for the sole purpose of providing a location to insert interference while measuring the loop. When placed in the position marked in Figure 2, the impedance of this resistor will affect the performance of the power supply. The optocoupler current must flow through this resistor and cause a regulation error. If you place a resistor in this position, a 0 ohm resistor must be used. A 50 ohm resistor can be temporarily placed to insert interference while performing loop measurements. Afterwards, the 0 ohm resistor must be replaced.

We have a large collection of isolated power supplies in our Power Lab library of reference designs. Here are some examples that include a 0 ohm placeholder resistor to measure the feedback loop in the position shown in Figure 2:

PMP9203 – Universal AC Input 5V/2A USB Adapter Reference Design with Constant Frequency Operation

PMP9204 – Universal AC Input 5V/2A USB Adapter Reference Design with DCM/Valley Switching and Optical Feedback

PMP9720 – 48V-60Vdc Input, 12V/150W Active Clamp Forward - Reference Design