Diode forward drop

Just to make things clearer, we have used some sample numbers in Figure 1-10. We have taken the applied input voltage to be 12 V and assumed a typical Schottky diode forward drop of 0.5 V. Note that we are assuming a perfect switch here (no forward drop), for the sake of simplicity. We make the following observations ... 

For a buck regulator, the duty cycle is (now including the switch and diode forward drops)... 

The rule-of-thumb is to pick a diode with a current rating at least equal to, but preferably at least twice the worst-case average diode current given below (for low losses, since the diode forward drop decreases substantially if its current rating is increased) ... 

The diode conduction loss is the other major conduction loss term in a power supply. It is equal to Vd x Id avg, where Vd is the diode forward-drop. Idavg is the average current through the diode — equal to Io for the boost and the buck-boost, and Io x (1 — D) for the buck. It too is frequency-independent. 

The temperature of DUT is determined by measuring the temperature-dependent parameter, the diode forward drop Vp). To minimize error, a four-point measurement is usually used. As shown in Figure 3.27, the temperature is inversely proportional to the forward drop (Vp). 

The amplitude of the SPICE model result has about a 1-V offset missing from the result. This is due to the forward drop of the zener diode, which is not typically modeled in zener diode models. It is not difficult to model this parameter, but since the purpose was to show the zero offset result, it is not important here. 

The only real error source for the output voltage are the forward drop characteristics of this diode. Each of the three SPICE simulators has a model for the 1N4002 diode, with all of the simulators within about 100 mV of each other. The question is, which model is correct The answer is they are probably all correct. The forward drop tolerance of a diode varies from lot to lot, from manufacturer to manufacturer, and from device to device. Table 10.1 shows the results of each of the three simulators along with the breadboard results. 

Assuming the 5 V output diode has a forward drop of 0.6 V, the turns ratio is... 

Assuming a 0.6 V forward drop across the diode, the required turns ratio is... 

Why did we say may above If we include the forward drops of the switch and diode in our calculation, we actually get a higher duty cycle than the 97% we got using the ideal equation D = Vq/Vin- The latter equation implicitly assumes Vsw = Vd = 0 (besides ignoring other key parasitics like the inductor s DCR). So the actual measured duty cycle in any application may well be a couple of percentage points higher than the ideal value. 

The essential difference from a conventional buck regulator is that the low-side mosfet in a synchronous regulator is designed to present a typical forward drop of only around 0.1 V or less to the freewheeling current, as compared to a Schottky catch diode which has a typical drop of around 0.5 V. This therefore reduces the conduction loss (in the freewheeling path) and enhances efficiency. 

We realize that the way to reduce conduction losses is by lowering the forward-drops across the diode and switch. So we look for diodes with a low drop — like the Schottky diode. Similarly, we look for mosfets with a low on-resistance Rds. However, there are compromises involved here. The leakage current in a Schottky diode can become significant as we try to choose diodes with very low drops. We can also run into significant body... 

During turn-off, the Drain current cannot change unless the switching node (and thus in effect Vd too) goes completely to Vin, and thereby forward biases the diode (ignoring its forward drop), allowing it to start sharing some or all of the Drain current Id. 

For the junction diode, the temperature coefficient K. of the forward drop Vf is ... 

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

A small Schottky rectifier with a current rating of about 20 to 30 percent of the MOSFET current rating (/d) is placed in parallel with the MOSFET s intrinsic P-N diode. The parallel schottky diode is used to prevent the MOSFET s intrinsic P-N diode from conducting. If it were allowed to conduct, it would exhibit both a higher forward voltage drop and its reverse recovery characteristic. Both can degrade its efficiency of the supply by one to two percent. 

Cutc/j diode. The eateh diode needs to be a sehottky diode to minimize the eon-duetion loss and the switehing loss of the funetion. The diode that has a reasonable forward voltage drop at the 3 A peak eurrent is the MBRD330 with a 0.45 V drop at 3 A (at -i-25°C). 

Synchronous Diode. A Sehottky diode about 30 pereent of the rating of the eon-tinuous rating of the synehronous MOSFET needs to be plaeed in parallel with the MOSFET s intrinsie diode. This would be about 0.66 A at 30 V. I will use an MBRS130. This diode produees a 0.35 V forward voltage drop at 0.66 A. 

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