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APPLICATION NOTE
MAGNETIC SNUBBER FOR 200W PFC WITH UNIVERSAL MAINS
by C. Adragna and N. Tricomi
In high voltage continuous mode boost converters, a significant part of the power mosfet switching losses is related to the turn-on edge. In fact, at turn on, the power mosfet has to sustain both the boost diode reverse recovery and the stray capacitances associated energies. Moreover, the additional peak current due to the recovery of the diode can be significantly high, in particular at high temperature, thus increasing the high frequency noise, the E.M.I. filter requirements and reducing efficiency. The turn on peak current, generating all the above mentioned problems, has been dramatically reduced by using the magnetic snubber we propose at Fig. 1b. The concept of this snubber is to reduce (and control) the turn-on di/dt of the mosfet to the most convenient value, considering the voltages and switching frequency applied to the system. Design Guidelines The dI/dt value can be selected so to reduce current-voltage overlap at turn-on but the main issue is generating a low level of EMI emission. On the other hand, the transition of the current in the switch from its peak value (Ipk) to zero has to be a small portion of the switch ON-time. A reasonable value is around 100A/s, which leads to Ls = 4H. The inductance value (Ls) is selected so to have the desired dI/dt. At MOSFET turn-on the voltage applied to the choke is Vout (until the boost diode has finished recovering): Vout L= (dI/dt) Figure 1a: Voltage Snubber.
T STTA506D Vo
The capacitor absorbs the energy stored in the choke and its voltage builds up from zero to a maximum (Vc) that can be found with energy balance considerations. This fixes the capacitance value. The resistor is selected so to have an RC constant much lower than the switch OFF-time. Part of the energy stored in the choke is dumped into the output but the majority is dissipated in the resistor so this must be properly rated. The diode (a 1A device is OK for 200W Pout) need not be a ultra fast recovery one since it does not conduct at MOSFET turn-on. It must be, instead, a fast turn-on type since it undergoes a very steep current peak. The RCD group clamps the extra voltage on the MOSFET's drain at turn-off, needed to reset the choke, so that the voltage rating of the MOSFET is not exceeded. The values mentioned in the test circuit (Fig. 1b) are related to the application 200W/100kHz UNIVERSAL MAINS "EVAL4981A". Figure 1a shows the voltage snubber circuit used in the same evaluation board (EVALL4981A). The benefits of this magnetic snubber are reported in the following table:
Vin 85 220 270 with voltage snubber Pin 232 220 217 88.4 93.2 94.4 with current snubber Pin 222 216 213 92.3 94.8 96.2 205 205 205 Pout
Analysing the table of results, it is possible to appreciate lower power dissipation. Figure 1b: Magnetic Snubber.
T STTA506D
Vo
CORE 77121 MAGNETICS
L=4H 8T
22/2W
1N4937
fsw=100KHz
STW/ STH15NA50 BYT11 600
270pF 630V
fsw=100KHz 33nF STW/STH15NA50
D95IN285C
1.8K 4W
D95IN307A
AN831/1297
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APPLICATION NOTE
This additional benefit, at 85Vac input mains, corresponds to about 10W, where the most of this reduced power dissipation is located in the power mos. There are three main benefits: reduction of currentvoltage overlap at switch turn-on (which reduces switching losses in the MOSFET), reduction of the reverse peak current in the diode (which means less power dissipation both in the diode and in the MOSFET: the latter "can't see" the recovery of the diode since it is isolated by the choke) and, last but not least, the controlled dI/dt leads to less EMI.
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APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGSTHOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics. (c) 1997 SGS-THOMSON Microelectronics - Printed in Italy - All Rights Reserved SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A.
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