Power Tip 43: Discrete devices – a good alternative to integrated MOSFET drivers (Part 2)
(Editor’s note: to see a linked list of all entries from #1 to the latest one, click here.)
In Power Tip #42, we discussed an emitter follower used in MOSFET gate-drive circuits and saw that drive currents in the 2-A range are achievable with small SOT-23 transistors. In this Power Tip, we look at self-driven synchronous rectifiers and discuss when discrete drivers are needed to protect the synchronous rectifier gates from excessive voltages.
Ideally, you would drive the synchronous rectifiers directly from the power transformer. However, with wide input voltage variations, the transformer voltage can be high enough to damage the synchronous rectifiers.
Figure 1 shows a discrete driver used to control the conduction of Q2 in a synchronous flyback. This circuit gives you controlled turn-on gate current and protects the rectifier gate from high reverse voltage.
Figure 1: Q1 quickly turns off synchronous flyback FET Q2.
The circuit starts with a negative voltage on the outputs of the transformer. The 12-V output is more negative than the 5-V output, causing Q1 to conduct and short the voltage gate-to-source on the power FET Q2, turning it off quickly. Since base current is flowing through R2, there is a negative voltage on speed-up capacitor C1.
During this time, the primary FET is conducting and stores energy in the magnetizing inductance of the transformer. When the primary FET turns off, the transformer output voltage swings positive. The gate-to-source of Q2 is rapidly forward biased through D1 and R1, causing Q2 to conduct. The Q1 base-to-emitter junction is protected by D2 when C1 discharges.
The circuit remains in this state until the primary FET is turned on again. Output current can actually discharge the output capacitors as a synchronous buck might do. Turning on the primary FET collapses the voltage on the transformer secondary and removes the positive drive from Q2. This transition can result in significant shoot through from overlapping the conduction time of the primary FET and Q2. To minimize the time when both primary and secondary FETs are on, Q1 shorts the gate-to-source on the synchronous rectifier Q2 as fast as possible.
Figure 2 shows a discrete driver used to control the conduction of Q1 and Q4 in a synchronous forward converter. In this particular design, the input voltage is wide-ranging. This means that the gates of the two FETs could be subjected to voltages beyond their ratings, so a clamping circuit is necessary.
Figure 2: D2 and D4 limit the positive gate voltage in this synchronous forward driver.
This circuit configuration turns Q4 on when the transformer output voltage is positive, and turns Q1 on when it is negative. Diodes D2 and D4 limit the positive drive to around 4.5 V. The FETs are turned off through D1 and D3, which are driven by the transformer and the current in the inductor. The reverse gate voltages are clamped to ground by Q1 and Q4.
In this particular design, the FETs have relatively small amounts of gate capacitance so the transitions are quick. Larger FETs may require implementing a PNP transistor to decouple the gate capacitance from the transformer winding and improve the turn-off speeds.
Selecting the proper package for gate drive transistors Q2 and Q3 is critical, as there can be considerable power dissipated in these transistors since they act as linear regulators during the charging of the FET gate capacitances. Additionally, with higher output voltages, the power dissipated in R1 and R2 can also be substantial.
To summarize, many power supplies with synchronous rectifiers can use the transformer’s winding voltage to drive the gates of the synchronous rectifiers. Wide-ranging inputs or high output voltages require conditioning circuits to protect the gates.
In the synchronous flyback illustrated in Figure 1, we showed how you can clamp the reverse voltage on the gate of the synchronous rectifier, while preserving fast switching transitions. Similarly, in the synchronous forward of Figure 2, we showed how you can limit the positive drive voltage on the gates of the synchronous rectifiers.
Please join us next month when we will discuss high di/dt load transients and their implications in designing and testing suitable power supplies.
For more information about this and other power solutions, visit: www.ti.com/power-ca.
About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.
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