Electrical – boost converter circuit simulation in LTSpice

The main power losses in a boost converter can be summarized as follows:

1. Power switch switching losses (e.g. MOSFET, BJT. Hereafter I will refer to the Power switch as the MOSFET)
2. MOSFET conduction losses.
3. DIODE switching losses.
4. DIODE conduction losses.
5. Other conduction losses (e.g. inductor resistance)

The efficiency of a converter is given by: eff = Po/Pin = (Pin - Plosses)/Pin.

As the losses change the efficiency therefore changes.

One can not make a blanket statement as to why the efficiency reduces or losses increase as the duty cycle increases because then one would need to know all five loss parameters as a function of current, voltage and switching frequency.

However, a simplified explanation of this phenomena is that the MOSFET conduction losses are unequal to the diode conduction losses. As the duty cycle increases, the MOSFET will conduct for a longer period and the diode for a shorter period. This in turn alters the power losses in the circuit. If the DIODE happens to have higher conduction losses than the MOSFET for example, then as the duty cycle changes, causing the DIODE to conduct for a relatively longer period than the MOSFET, then the efficiency will decrease. This is a simplified explanation, but the main principle is that as you change the duty cycle, the operating conditions for each element in the circuit change. Since the losses for each device depend on it's specific operating point, then changing the duty cycle changes the losses.

The S1JB-13-F diode has a pretty slow reverse recovery time (trr is 1.8us typ, 3us max). The boost switcher appears to be variable frequency, but at some loads it's switching frequency may be uncomfortably close (or right on top of) this reverse recovery time.

The MIC2251 data sheet recommends a Schottky diode but points out a fast switching diode can be used (their recommendation is the LS4148, whose trr is less than 8ns).

See this question for more on diode reverse recovery time.

A boost converter works by grounding the SW node during the 'on' time, which charges up the inductor. When the converter switches the internal FET off, the current flowing through the inductor needs to go somewhere, and the diode is preferred. So the voltage rises until the SW node is a diode drop above the desired output voltage. When the inductor has dumped its energy into the output cap/load, the diode needs to turn completely off. While it's turning off, you will get current flowing in the reverse direction, and depending on how long that recovery time is. The longer it is, the more time there is for current to flow backward and cause heating. In the worst case, if the switching frequency lines up badly, you can have the FET turning on again before the diode has recovered. I suspect that is what was happening in your case, since the converter was also exhibiting some temp rise and eventual failure.

You can see a couple examples of the operating frequency and switching waveforms in the graphs on Page 5 of the MIC2251 data sheet. And there is a graph of operating frequency vs load current on Page 4.