On the output voltage of a real flyback converter

13.05.2018 13:18

I was recently investigating a small switch-mode power supply for a LED driver. When I was looking at the output voltage waveform on an oscilloscope it occurred to me that it looked very little like the typical waveforms that I remember from university lectures. So I thought it would be interesting to briefly explain here why it looks like that and where the different parts of the waveform come from.

The power supply I was looking at is based on the THX203H integrated circuit. It's powered from line voltage (230 V AC) and uses an isolated (off-line) flyback topology. The circuit is similar to the one shown for a typical application in the THX203H datasheet. The switching frequency is around 70 kHz. Below I modified the original schematic to remove the pi filter on the output which wasn't present in the circuit I was working with:

Schematic of a flyback converter based on THX203H.

When this power supply was loaded with its rated current of 1 A, this is how the output voltage on the output terminals looked like on an oscilloscope:

Output voltage of a flyback converter on an oscilloscope.

If you recall the introduction into switch mode voltage converters, they operate by charging a coil using the input voltage and discharging it into the output. A regulator keeps the duty cycle of the switch adjusted so that the mean inductor current is equal to the load current. For flyback converters, a typical figure you might recall for discontinuous operation is something like the following:

Idealized coil currents and output voltage in a flyback converter.

From top to bottom are the primary coil current, secondary coil current and output voltage, not drawn to scale on the vertical axis. First the ferrite core is charged by the increasing current in the primary coil. Then the controller turns off the primary current and the core discharges through the secondary coil into the output capacitor.

Note how the ripple on the oscilloscope looks more like the secondary current than the idealized output voltage waveform. The main realization here is that the ripple in this case is defined mostly by the output capacitor's equivalent series resistance (ESR) rather than it's capacitance. The effect of ESR is ignored in the idealized graphs above.

In this power supply, the 470 μF 25 V aluminum electrolytic capacitor on the output has an ESR somewhere around 0.1 Ω. The peak capacitor charging current is around 3 A and hence the maximum voltage drop on the ESR is around 300 mV. On the other hand, ripple due to capacitor charging and discharging is an order of a magnitude smaller, at around 30 mV peak-to-peak in this specific case.

Adding the ESR drop to the capacitor voltage gives a much better approximation of the observed output voltage. The break in the slope marks the point where the coil has stopped discharging. Before that point the slope is defined by the decaying current in the coil and the capacitor ESR. After that point, the slope is defined by the discharging output capacitor.

ESR voltage drop, capacitor voltage and output voltage in a flyback converter.

The only feature still missing is the high-frequency noise we see on the oscilloscope. This is caused by the switching done by the THX203H. Abrupt changes in the current cause the leakage magnetic flux in the transformer and the capacitances of the diodes to oscillate. Since the output filter has been removed as a cost-cutting measure, the ringing can be seen unattenuated on the output terminals. The smaller oscillations are caused by the primary switching on, while the larger oscillations that are obscuring the rising slope are caused by the THX203H switching off the primary coil.

Switching noise on the output voltage of a flyback converter.

Posted by Tomaž | Categories: Analog

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