Puzzler 1

Why?:

In selecting the output rectifier three key parameters should be considered; voltage rating, current rating and reverse recovery time.

Considering each of these in turn we can see which of the diodes is suitable.

Maximum repetitive reverse voltage (V_RRM)

Ideally the diode selected should have a V_RRM rating 1.25·V_R, where V_R is the reverse voltage seen by the diode. The 1.25 de-rating factor provides margin to take account of leakage inductance generated voltage spikes, and AC line transients that increase the DC bulk voltage temporarily plus reducing the device stress improves reliability.

V_R = V_O + (V_MAX)·(N_S/N_P)

= 12 + 375·6/58

Applying the 1.25 V de-rating this gives a minimum diode rating of 63 V. The closest standard diode rating would be an 80 V Schottky or a 100 V PN diode.

Therefore all the diodes have an acceptable V RRM rating except (c) and (e).

Forward Current Rating (I_AVE)

Diodes manufacturers generally specify the current rating as the average current through the diode or for a Flyback power supply the load current. In this design the specified output current is 2.5 A so a diode with a current rating above 2.5 A is acceptable right? Yes and no.

• The diode manufacturers provide de-rating curves so for example at a lead temperature of 100 °C a 3 A PN diode is actually a 1.5 A diode!
• Diode type. The choice between PN and Schottky and the actual voltage rating changes a diode’s forward voltage. The lower the forward voltage drop, the lower the dissipation is for a given forward current.
• Overload fault conditions. All power supplies have some degree of overload capability. Due to tolerances a typical supply will be able to deliver significantly more than the specified output power, especially at high line. Therefore if an overload condition can exist then the diode should be sized to operate without overheating and failing. Products from Power Integrations significantly help here as the key parameters associated with power delivery (frequency and current limit) are very tightly specified, reducing the overload power. In addition line voltage power limiting is easy to add to reduce overload power. In this example the addition of R2 reduced the primary current limit as the line voltage increases and provides a very flat overload power characteristic with line voltage as shown below.

As a general rule of thumb select the current rating such that I_AVE = 3 x I_O irrespective if the diode is a PN or a Schottky type (although a Schottky diode will have lower dissipation check the maximum operating temperature – some are 25-50 ° C lower than a PN diode). Finally measure the diode temperature under maximum overload power, highest ambient to check it is within either internal or manufacturers design limits.

Looking at the diodes in the list to determine if they are acceptable or not:

So diodes (a), (d) and (e) have an acceptable current rating but if you ruled out (a) that would also be ok.

Reverse recovery time (t_rr)

An ideal diode would instantly block reverse current flow when a reverse bias is applied. In practice a Schottky approaches zero recovery time however for a PN diode it takes a finite time for charge stored in the diode to be swept away before it can block. The amount of charge is proportional to the current flowing through the diode.

This is significant as even a Flyback converter operating in discontinuous conduction mode will actually operate in continuous conduction mode at start-up meaning the output diode has to reverse recover with a significant forward current flowing. In this design the converter operates in continuous conduction mode some the diode has to recover with forward current on every switching cycle.

To prevent large reverse currents from flowing through the diode when the MOSFET in U1 turns on, applying a reverse voltage, the diode should have a reverse recovery time less than switching time. This means a recovery time of 50 ns or better.

Longer duration than this causes large primary side current spikes at the turn on event and high diode dissipation – for example the 1N5400 became hot enough to melt the solder holding into the PCB while taking the measurements for question 2.

So diodes (a), (d) and (e) have an acceptable reverse recovery time.

Why?:

As discussed for question 1, as the reverse recovery time of the diode increases so does the magnitude of the reverse current. This current is transformed according to the turns ratio of the transformer and results in a large drain current spike when the TOPSwitch turn on. Accordingly the slowest diode, the 1N5400 causes the largest spike – large enough to trigger the internal TOPSwitch current limit at the end of the 220 ns leading edge blanking time.

This waveform also shows the effect of the diode forward current on reverse recovery time. The next switching cycle occurs normally with almost no current spike, as there is no current flowing in the diode when it is reverse biased. As no energy was stored in the transformer due to the early termination of the switching cycle all the energy in the transformer is transferred to the load and the diode current falls to zero.

Next is sequence is the FR257, and the UF5406 both which have unacceptably high reverse currents. With a turns ratio of 58:6 the FR257 allowed a reverse current of 15.4 A (1.6 x 58/6) to flow during reverse recovery. This explains why this and all the slow diodes would have been destroyed had the test been longer than a few seconds.

The 25 ns reverse recovery of the BYV29-100 diode produces identical results to the Schottky – the spike now seen is purely due to parasitic capacitance, largely the capacitance of the primary winding.

Diode reverse currents effectively recover some of the leakage inductance energy stored in the capacitance of the clamp. The longer the recovery time, the more energy is recovered.

Why?:

The two oscillograms below show the drain voltage (upper, 200 V/div), drain current (lower, 0.4 A/div) and the current through the clamp diode, D1, for both a UF4005 and 1N4007GP (middle, 0.4 A/div).

The slower recovery time of the 1N4007GP causes a reverse current to flow back through the primary winding at the end of the clamping period. This partially discharges the leakage energy stored in clamp capacitor, C2, and transfers some of the energy stored to the secondary, thus improving efficiency.

It might appear that the ideal diode would be very slow to maximize this effect. However, to prevent excess diode dissipation, to limit drain ringing for EMI reasons, and to prevent the drain ringing below the source at low line, the diode must have a specified recovery time. A small value series resistor is also required to limit ringing. In this case R3 acts as the damping resistor. This resistor can also be placed directly in series with D1. The 1N4007GP is a glass passivated version of the 1N4007, with a reverse recovery time specified at 2 us. The 1N4007 does not have a specified recovery time and must not be used.

UF4005

1N4007GP

Reverse recovery current at the end of the clamp period recycles stored clamp energy to the secondary