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1 INTRODUCTION
Where high-voltage bipolar transistors are used in off-line flyback or other converters, stress voltages of the order of 800 V may be encountered. High-voltage transistors with Vceo ratings in the range 400 to 1000 V generally behave somewhat differently from their lower-voltage counterparts. This is due to a fundamental difference in the construction of high-voltage devices.
To obtain the most efficient, fast, and reliable switching action, it is essential to use correctly profiled base drive current waveforms. To explain this, a simplified review of the physical behavior of high-voltage bipolar transistors would be useful. (A full examination of the physics of high-voltage transistors is beyond the scope of this guide) High-voltage devices generally have a relatively thick region of high-resistivity material in the collector region, and low-resistance material in the base-emitter region. As a result of this resistance profile, it is possible (with an incorrectly profiled base drive) to reverse-bias the base-emitter region during the turn-off edge. This reverse-bias voltage effectively cuts off the base-emitter diode, so that transistor action stops. The collector current is now diverted into the base connection during the turn-off edge, giving diode like turn-off switching action. That is, the collector-base region now behaves in the same way as a reversed-biased diode. It displays a slow recovery characteristic and has a large recovered charge.
2 SECONDARY BREAKDOWN
The slow recovery characteristic described above is particularly troublesome during the turn-off edge with inductive collector loads (such as would be presented by the normal leakage inductance of a power transformer).
As a result of the current forcing action of the collector inductance, any part of the chip that remains conducting during a turn-off edge must continue to carry the previously established collector current. Hence, the slow blocking action of the reverse-biased diodelike collector-base recovery not only results in slow and dissipative turn-off, but also gives rise to "hot spots" on the chip as the current is forced into a progressively small conduction area during the turn-off edge.
It is these "hot spots" that overstress the chip and may cause premature failure. The effect is often referred to as "reversed-biased secondary breakdown.
3 INCORRECT TURN-OFF DRIVE WAVEFORMS
Surprisingly, it is the application of energetic and rapid reverse base drive during the turn off edge that is the major cause of secondary breakdown failure of high-voltage transistors with inductive loads.
Under aggressive negative turn-off drive conditions, carriers are rapidly removed from the area immediately adjacent to the base connections, reverse-biasing the base-emitter junction in this area. This effectively disconnects the emitter from the remainder of the chip. The relatively small high-resistance area in the collector junction will now grow relatively slowly (1 or 2 µs), crowding the collector current into an ever-diminishing portion of the chip.
As a result, not only will the turn-off action be relatively slow, but progressively increasing stress is put on the conducting region of the chip. This leads to the formation of hot spots and possible device failure, as previously explained.
4 CORRECT TURN-OFF WAVEFORM
If the base current is reduced more slowly during the turn-off edge, the base-emitter diode will not be reverse-biased, and transistor action will be maintained throughout turn-off.
The emitter will continue to conduct, and carriers will continue to be removed from the complete surface of the chip. As a result, all parts of the chip discontinue conducting at the same instant.
This gives a much faster turn-off collector-current edge, gives lower dissipation, and eliminates hot spots. However, the storage time (the delay between the start of base turn-off and the collector-current edge) with this type of drive will be longer.
5 CORRECT TURN-ON WAVEFORM
During the turn-on edge, the reverse of the above turn-off action occurs. It is necessary to get as much of the high-resistance region of the collector conducting as quickly as possible. To achieve this, the base current should be large, with a fast-rising edge; thus carriers are injected into the high-resistance region of the collector as quickly as possible.
The turn-on current at the beginning of the "on" period should be considerably higher than that necessary to maintain saturation during the majority of the remaining "on" period.
6 ANTISATURATION DRIVE TECHNIQUES
To reduce the storage time, it is a good practice to inject only sufficient base current toward the end of the "on" period to just ensure that the transistor remains near but not into saturation. Self-limiting anti-saturation networks ("Baker clamps") are recommended for this.
With inductive loads, in addition to the base-current shaping, it is usually necessary to provide "snubber networks" between collector and emitter. This snubbing also helps to prevent secondary breakdown.
It should be remembered that low-voltage power transistors will not necessarily display the same behavior. These transistors often have a much more heavily doped collector region, and the resistance is much lower. Applying a rapid reverse-bias voltage to these devices during turn-off is unlikely to generate a high-resistance area. Hence, with low voltage transistors, fast switching action and short storage times are best achieved by using fast reverse-biased base drive during the turn-off edge.
7 OPTIMUM DRIVE CIRCUIT FOR HIGH-VOLTAGE TRANSISTORS
A fully profiled base drive circuit is shown in FIG. 15.1a, and the associated drive wave forms are shown in FIG. 15.1b. This drive circuit operates as follows.
When the drive input to point A goes positive, current will initially flow via C1 and D1 into the base-emitter junction of the switching transistor Q1. The initial current is large, limited only by the source resistance and input resistance to Q1, and Q1 will turn on rapidly.
As C1 charges, the voltage across R1, R2, C2, and Lb will increase, and current will build up in Lb during the remainder of the "on" period.
Note: While current is flowing in Lb, C2 will continue to charge until the voltage across it equals the zener voltage (D2). D2 now conducts, and the drive current will be finally limited by R1. (R2 has a relatively large resistance, and the current flow in R2 is small.) When the drive goes low, D1 blocks, and C1 discharges into R2. The forward current in Lb decays to zero and then reverses under the forcing action of the reverse voltage at point B. (C2 is large and maintains its charge during the "off" period.) Hence, during the turn-off edge current builds up progressively in the reverse direction in the base-emitter of Q1 until the excess carriers are removed and the base-emitter diode blocks. At this instant the voltage on Q1 base flies negative under the forcing action of Lb, forcing the transistor into reverse base-emitter breakdown. This reverse breakdown of the base-emitter diode is a non-damaging action and clamps off the base emitter voltage at the breakdown value until the energy in Lb has been dissipated.
The base drive current waveforms are shown in FIG. 15.1b.
Although it is not essential to profile the drive current waveform for all types of high voltage transistor, most types will respond well to this type of drive. If the selected transistor is not rated for reverse base-emitter breakdown, then the values of Lb and R3 should be selected to prevent this action, or clamp zeners should be fitted across the base-emitter junctions.
Since switching device secondary breakdown is probably the most common cause of failure in switch mode power supplies, the designer is urged to study appropriate references.
FIG. 1 (a) Base drive current shaping for high-voltage bipolar transistors.
(b) Collector voltage, collector current, base drive current, and base
emitter voltage waveforms.
8 QUIZ
1. Why do some high-voltage bipolar transistors require specially profiled base drive cur rent waveforms?
2. Explain one cause of secondary breakdown in a high-voltage bipolar transistor.
3. Draw the typical ideal base drive current waveform for high-voltage bipolar transistors with inductive loads.
Also see: Our other Switching Power Supply Guide