Design Con 2015

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

Paul Lacey -January 15, 2013

In Part one of this article, we discussed testing low voltage operation, testing full load and measuring peak drain voltage.

Calculating Transformer Primary Inductance

Next, turn off the AC input and connect the high-voltage oscilloscope probe across the terminals of the input bulk capacitor. Then apply the minimum AC input to your supply and load the output(s) to full load.  Configure the scope to view both the DC bus voltage, seen by the high-voltage probe connected across the bulk capacitor, and the drain switching current.

Using the scope measure the di/dt ratio of the drain current during the most linear portion of the ramp. Typically this occurs between 25 and 75 percent of the current limit. At the same time measure the average DC bus voltage observed during the same interval. With these two pieces of information, you can calculate an approximate value of the primary inductance of the transformer using the basic relationship of inductors:  V = L ∆i/∆t.

 

Figure 3: Calculate transformer primary inductance by measuring the di/dt ratio of the drain current during the most linear portion of the ramp and the average DC bus voltage during the same period.

When the MOSFET is turned on, the voltage seen across the transformer primary is approximately equal to the average DC bus voltage. The current through the inductor is the same as the drain current. To rewrite this equation; L= V ∆t/∆i. Calculate the value of L and compare it to the value specified in your design. If you are using a device from Power Integrations, you can use the PI Expert tool to quickly determine this value. If the value exceeds the device’s defined tolerances, contact your transformer manufacture.

The next step is to check for high initial current which occurs immediately after the MOSFET is switched on. Turn off the AC source and reconnect the high-voltage oscilloscope probe across the MOSFET to measure drain switching voltage. Apply the maximum specified AC input voltage and load your supply to full load. Configure the oscilloscope to view both the MOSFET voltage and current and trigger on the rising edge of the drain voltage. Set the time base just wide enough to monitor one complete switching cycle.

When you look at the turn-on edge of the drain current waveform, you may see a spike of current. The spike is caused by parasitic capacitances which quickly discharge through the MOSFET and is normal in switching power supplies.

Some devices, such as those from Power Integrations, offer a function which disables the current limit sensor for a fixed period of time following MOSFET turn-on. This leading-edge blanking feature prevents the initial current spike from triggering the current limit and prematurely terminating the current pulse. However, if the turn-on spike is larger than normal, it can still trigger the initial current limit of the device and cause it to limit power transfer to the output.

 

Figure 4: If the current through the MOSFET after the leading-edge blanking time exceeds the initial current limit, it may cause power delivery problems.

 

Determine the minimum leading-edge blanking time from the datasheet for the device you are using in your design. Then measure the current level you see through the MOSFET after the leading-edge blanking time. Compare this level with the initial current limit found on the device datasheet. If the value you measured on the MOSFET exceeds the initial current limit, you may experience power delivery problems in your design.

Repeat this measurement at the lowest specified input voltage. If the design operates in continuous conduction mode at low line, the initial current pedestal will add to the initial current spike.

Bias Winding Voltage

For designs using a bias winding, turn off the AC input and connect an oscilloscope voltage probe, set to measure DC voltage, across the filter capacitor for the bias winding output. If needed, solder two short leads to the back of the board as test points. Then apply the minimum AC input and remove all loading from the supply output.

Use the oscilloscope to measure the lowest voltage seen across the bias winding capacitor over an entire cycle. Generally we recommend the lowest bias winding voltage to fall somewhere between 8 V and 9V. If the lowest bias winding voltage is < 8 V, it can create regulation problems in your design. You will need to add more turns to your bias winding to increase the voltage.

To avoid adding too much voltage, we suggest adding one turn at a time and then retesting the voltage in your design. Too many turns may boost the bias winding voltage excessively and in the process increase no-load dissipation in your design. In some designs, an increase in the value of the bias winding filter capacitor can provide sufficient hold-up to pull the lowest bias winding voltage above 8 V.


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