Power Supply Industry News Knowledge is Power ™


 Home   Design Tools   Articles   Resources   My SMPS   Register   Login   Designers   Manufacturers   Lawyers 


Summary MOSFET Switching: Part I - Turn-ON, Hard Switching 
Category Other 
Subcategory MOSFET Switching 
Power
Supply
Components
This article is trying to make sense out of confusing information regarding the behavior of a MOSFET during switching sequences, in numerous technical articles.

We are not attempting to explain the physics behind a MOSFET structure. For those interested to find more about a MOSFET structure, we recommend the SGS-Thomson technical articles mentioned in the references. The purpose of the article is to present a power supply design engineer with facts that will help design a MOSFET driving circuit, calculate the estimated losses for critical events, predict the efficiency of a power supply, estimate the junction temperature for critical components and various stresses, and ultimately, helping make decision to optimize a design.

The MOSFET switching events are analyzed for an inductive load, diode clamping circuit, the only one that applies to a switching power supply. The datasheet information or technical articles regarding resistive loads have little or no relevance to switching a MOSFET in a switch mode power supply. Also the article is considering only 500V/600V MOSFETs, most relevant and for switch mode power supplies.

And now the comments:
-Gate voltage has a sharp rise at t1 due mainly to the MOSFET source inductance.
-Internal MOSFET channel will carry also the Coss discharge current.
-At t3 the drain voltage will reach Vx, usually around 25V for 500V MOSFETS. After this time Vds*Idrain loss should be considered part of the conduction loss.
-Q3, gate charge associated with drain voltage reaching Vx, is much smaller then Q3+Q4, commonly specified in a MOSFET datasheet.

Common errors and misconceptions:

Error: Drain voltage will decreased linearly to zero during t2 - t4 period, when gate voltage is constant (gate plateau voltage).
Reality: Drain voltage will decrease much faster reaching Vx voltage, during t1 - t2 period. Calculating the switching loss associated with this period of time, considering that the drain voltage will decrease linearly for the entire "plateau" period, will results in huge errors.

Error: Drain voltage will reach zero at t2b (the end of the diode reverse recovery period).
Reality: Drain voltage will decrease much faster reaching Vx voltage, during t1 - t2 period. Calculating the switching loss associated with this period of time, considering that the drain voltage will decrease linearly for the entire "plateau" period, will results in huge errors.

Error: MOSFET capacitances cannot be used to determine switching behavior, you need gate charges values.
Reality: MOSFET capacitances, if fully characterized, can fully explain (together with other parameters) the switching behavior, without the need for gate charges.
 
Webpage More info 
Author SMPS Editor 
Date 1/7/2010 
 
 

2700 currently registered users receiving weekly email news. 7000 registered since 2002.
Copyright © SMPS Power Supplies™. All rights reserved.