Bringing up a flyback supply for the first time, Part one

No test procedure offers more potential for surprise and, therefore, more apprehension for power supply designers than powering up a new design for the first time.  In far too many cases designers find they inadvertently damage components or discover hidden design flaws. The following article offers a step-by-step process for safely performing this important task and ensuring your supply is functioning correctly. It also offers helpful links to additional information for troubleshooting specific problems within your design. 

To conduct the tests described in this article you will need an isolated AC source and Variac, a wattmeter, at least four multi-meters including two offering a high resolution current range, an oscilloscope with a high voltage probe, a current probe, an electronic load and your actual load. Test time will take approximately one to two hours. Finally, as a precaution, we recommend you only conduct these tests if your design is constructed on a printed circuit board that meets the layout guidelines defined by your device manufacturer.

Using a breadboard or proto-board can introduce undesirable parasitic elements into the circuit that may prevent proper operation. In addition, many breadboards are not designed to carry the current levels generated by a switching power supply and make it extremely difficult to control creepage and clearance distances.

Testing Low Voltage Operation

Begin the test process with a simple visual inspection of the board to ensure that all polarized components have been properly inserted. While it is a relatively rare occurrence, an incorrectly-inserted component can cause a destructive failure.

The next step is to check the power supply’s operation at low voltage input. To accomplish this task, you must disable the undervoltage lockout feature if it is enabled. Typically, this requires removing the UV resistors from the board. In the sample circuit depicted below, the UV resistors are connected between the DC bus and the M pin of Power Integrations’ TOPSwitch-HX device, an integrated, high efficiency power conversion IC incorporating a 700 V power MOSFET for use in flyback power supplies.

In this case you must remove the resistors highlighted in the schematic and short the M pin to source. If you are using a different device you can usually identify the correct components and method for disabling the UV function using the device datasheet.

Fig. 1 To test low voltage operation, disable undervoltage lockout by removing the UV resistors highlighted in the schematic below. Source: Power Integrations

Next, solder the two short leads to the negative and positive terminals of the input capacitor to act as test points.  To properly confirm low voltage operation, you will need to monitor the output voltage and DC bus voltage across the input capacitor while applying a low AC input voltage. Connect a multi-meter across the output terminals of the board and another across the input capacitor using the two test points. Both meters should be set to read DC voltage.

If your design uses multiple outputs, connect load resistors to any outputs other than the main regulated output. Make sure the load resistors are sized to draw the minimum load specified for each output. This prevents these output voltages from rising outside of specification due to peak charging. If no minimum load is specified for an output, then select the resistor to draw an output current of 5 mA.

Connect the AC input leads to the board. Check to ensure the AC input is correctly connected to the power supply input terminals and not the DC output. Improperly connecting the AC input can severely damage the supply.

This test also requires you to measure AC input power. Set up a wattmeter in the AC input path and configure it to measure AC voltage, current and input power as outlined in the operator’s manual. If you don’t have a wattmeter, connect a third multi-meter in series with the AC input (set to measure AC current) and a fourth across the power supply input terminals to measure AC voltage.

Next, ensure the Variac or AC source is set to zero and turn it on. Slowly increase the input voltage to about 10 VAC. The AC input voltage on either the wattmeter or input multi-meter should increase. If it does not, check that your AC source is properly configured. You should also see the voltage on the DC bus increase as you apply AC voltage.