Analysis of the Loss of Power Bipolar Transistors Due to Electron Irradiation

Power bipolar transistors have been widely used as power switch tubes in switching power supplies due to their low cost. The application of electron irradiation technology can reduce the minority carrier lifetime, reduce the storage time and fall time of power bipolar transistors, increase the switching speed, and have good consistency and repeatability, and high yield rate, which is unmatched by the traditional manufacturing process of high reverse voltage power switch transistors. In order to reduce the loss of power bipolar transistors, this paper uses 10 MeV electron irradiation to reduce its turn-off delay time and improve the conversion efficiency of the switching power supply.

By adding a clamping circuit to the power bipolar transistor, the transistor cannot reach deep saturation and the turn-off delay and turn-off loss can be reduced. This paper also compares the electron irradiated bipolar transistor and the clamped bipolar transistor.

The switching power supply used in this experiment is the 3765 series charger developed by BCD Semiconductor, and the power bipolar transistor used is the APT13003E provided by BCD Semiconductor, which is widely used in power switching circuits such as electronic ballasts, battery chargers and power adapters.

1 Losses of switching transistors in switching power supplies

Figure 1 shows a typical flyback switching power supply schematic. In the schematic, the collector of the switching transistor Q1 is connected to the transformer T1. When the controller is driven to a high level, Q1 is turned on and energy is stored in the transformer T1. When the controller is driven to a low level, Q1 is turned off and energy is released to the back end through the transformer T1. Figure 2 shows a schematic diagram of the collector voltage and current waveforms during the switching process of the switching transistor.

The loss of the transistor during operation is divided into switching loss and steady-state loss, of which switching loss includes conduction loss and turn-off loss, and steady-state loss includes conduction loss and cut-off loss. The cut-off loss accounts for a very small proportion of the total loss and can be ignored. We define the time taken for Vce to drop from 90% Vindc to 110% Vcesat as the turn-on delay, i.e. t1 - t0 in Figure 2, and the time taken for IC to drop from 90% Icmax to 0 as the turn-off delay, i.e. t3 - t2.

When the switching transistor is turned on, the collector voltage increases when the controller driving voltage is high, the base current increases, and the collector voltage drops from Vindc to 0. At this time, since the voltage difference across the parasitic capacitor in parallel with the primary side of the transformer also changes from 0 to Vindc, the parasitic capacitor is charged, so a peak current is generated in the collector of the switching transistor. On the other hand, if the reverse recovery current of the secondary rectifier diode does not drop to 0, this peak current will be further increased. The switching transistor has an alternating collector voltage and current phenomenon, resulting in conduction loss until the collector voltage drops to Vcesat. The conduction loss can be expressed as:

After the transistor is turned on, the collector current gradually increases from 0, but Vcesat is not 0, so conduction loss occurs. The conduction loss can be expressed as:

When the switching transistor is turned off, the collector current cannot drop to 0 immediately, but the collector voltage has already started to rise from Vcesat, resulting in voltage and current alternation on the switching transistor, thereby causing turn-off loss.

Since the transformer is an inductive element, when the switch is suddenly turned off, the current of the transformer inductive element cannot change suddenly, which will generate a large flyback voltage, hindering the current change, and adding to the switch tube through the circuit, resulting in relatively large losses. The turn-off loss can be expressed as:

The total loss of the switch can be expressed as:

Generally speaking, turn-off loss accounts for the largest proportion of switching loss, and turn-off loss is related to the turn-off delay time of the switching transistor. Reducing the turn-off delay time (t3 - t2) and speeding up the collector current drop rate can reduce the total loss of the switching transistor.

2 Electron irradiation experiment

Electron irradiation can introduce a variety of deep energy levels in silicon. These energy levels will contribute to the recombination of non-equilibrium carriers according to their position in the bandgap, the size of the electron-hole capture cross-section and the size of the energy level density, thereby causing a decrease in the minority carrier lifetime and carrier concentration, thereby affecting some parameters related to the minority carrier lifetime, such as the switching time of the transistor and the current gain factor (hFE).

In the experiment, we divided the unpackaged power bipolar transistor APT13003E wafers into four groups. The first group was used as the control group and was not irradiated. The other three groups were irradiated with 10M eV electrons, with irradiation doses of 5 kGy, 10 kGy, and 15 kGy, respectively. After irradiation, they were annealed at 200℃ for 2 h, and then the four groups of wafers were packaged to become finished products. Table 1 shows the FT test results of the four groups of transistors.

Table 1 FT test results of four groups of APT13003E

From Table 1 we can see that after irradiation, the storage time ts decreases significantly with the increase of irradiation dose, the fall time tf decreases, and the rise time tr increases; the current amplification factor decreases with the increase of irradiation dose; the saturation voltage drop and breakdown voltage HBVceo increase with the increase of irradiation dose.

3 System Test Results

Four different sets of APT13003E switching transistors were placed in the same AP3765 charger system developed by BCD Semiconductor. The charger has a power of 3W, an input AC voltage range of 85V to 264V, and an output DC voltage of 5V. Figure 3 shows the average system efficiency increase of the electron-irradiated APT13003E and the conventional APT13003E at output load currents of 0.15A, 0.30A, 0.45A, and 0.60A (i.e., 25%, 50%, 75%, and 100% load) at 85V, 115V, 230V, and 264V AC input voltages.

Figure 3 The percentage increase in system average efficiency of the electron-irradiated APT13003E and the conventional APT13003E at various AC input voltages

As can be seen from Figure 3, at lower AC input voltages (such as 85 V and 115 V), the system efficiency of the irradiated APT13003E is higher than that of the unirradiated APT13003E. However, at higher AC input voltages (such as 230 V and 264 V), the irradiated APT13003E fails to improve the system efficiency. Under 85 V AC input voltage, APT13003E with 10 kGy irradiation dose has the best performance. The total loss of the switching transistor is reduced from 0.209 W to 0.121 W, a decrease of 42%, which increases the overall efficiency of the system by 2.1%. If the switching transistor adopts TO-92 package, the junction temperature of the switching transistor will be reduced by about 11 °C. Under 115 V AC voltage, the overall efficiency of the system is also improved by about 1.4%, and the junction temperature of the switching transistor will be reduced by about 7 °C, which effectively improves the reliability of the switching transistor and reduces the loss of the switching power supply.

When the irradiation dose is further increased to 15 kGy, the improvement in system efficiency decreases instead. Therefore, in order to obtain the best system efficiency, the most appropriate irradiation dose needs to be used.

We tested the collector voltage and current waveforms of four groups of APT13003E under the conditions of 85 V and 264 V AC input voltage and 0.45 A output current, and analyzed the losses in each stage of the switching transistor operation. The results are shown in Table 2. tON represents the turn-on delay, toff represents the turn-off delay, Tw is the switching period, Pin is the charger input power, Plos STot is the total loss of the switching transistor, and Ploss tot /Pin is the percentage of the switching transistor loss in the system input power.

Table 2 Loss analysis of four groups of APT13003E in each stage of the charger system

As can be seen from Table 2, under 85 V AC input voltage, the turn-off delay of the irradiated APT13003E is significantly reduced compared to the unirradiated APT13003E, so the turn-off loss is greatly reduced. For example, the turn-off loss of the tube irradiated with 10 kGy is reduced to 1/6 of the unirradiated tube; the turn-on delay increases, but the increase is small, and the conduction loss increases slightly; the saturation voltage drop increases with the increase of irradiation dose, so the conduction loss increases with the increase of irradiation dose. The increase of turn-on loss and conduction loss and the reduction of turn-off loss are a contradiction, so the appropriate irradiation dose must be selected to minimize the total loss of the switching transistor.

At an input voltage of 264 V, the turn-off loss after irradiation is only slightly reduced, so the total loss remains basically unchanged and the system efficiency is not improved. Figures 4 and 5 show the waveforms of the base current, collector voltage and current of the unirradiated APT13003E at input voltages of 85 V and 264 V, respectively. Comparing Figure 4 and Figure 5, it can be seen that the peak of the collector current when turned on at 264 V input voltage is much larger than that at 85 V. This is because the charging voltage of the transformer parasitic capacitance increases by 2.1 times when turned on, but the charging time only increases by about 0.6 times, so the charging current will increase greatly, which also causes the conduction loss of APT13003E to change from 0.016W at 85 V to 0.183W at 264 V. At this time, the conduction loss accounts for most of the total loss, and electron irradiation does not improve the conduction loss; on the other hand, when APT13003E is turned off, the collector voltage does not drop directly to 0, but first drops to 0 after a current "tail" of nearly 100 ns. At this time, the collector voltage is already relatively large, so the loss caused by this current "tail" accounts for a large proportion of the turn-off loss. The reason for this "tail" is that when the switching transistor is turned off, since the base region of the tube is relatively thin, the excessive base current causes a large base potential difference, making the emitter partially forward biased when VBE is negative, and the collector current takes a long time to drop.

Figure 4 Waveforms of base current, collector voltage, and collector current of APT13003E at 85 V AC input voltage

Figure 5 APT13003E base current, collector voltage, and collector current waveforms at 264 V AC input voltage

After electron irradiation, the "tail" of the collector current of APT13003E has not decreased, so the turn-off loss of APT13003E after irradiation has not been greatly reduced, and the efficiency of the system has not been improved. On the one hand, we can optimize the base drive circuit so that the base reverse current is not too large at the beginning of the turn-off to avoid the generation of the current "tail", and the reverse base current is increased suddenly in the final stage of the turn-off. Under high input voltage, the efficiency of the system will be improved; on the other hand, by means of segmented winding, using insulating materials with small dielectric constants, appropriately increasing the thickness of the insulating layer and electrostatic shielding, the parasitic capacitance of the transformer is reduced, the conduction loss of the switching transistor is reduced, and the system efficiency will also be improved.

Comparison between electron irradiation tube and clamped switching tube

The use of clamped switching transistors can also reduce the turn-off delay of the switching transistor. The principle is that the clamping circuit prevents VBC from increasing to 0.7 V required for deep saturation when the transistor is turned on, so that the minority carriers injected on both sides of the collector junction are very few, so that the excess stored charge is very small, and the storage time is greatly shortened. There are two main types of clamped switching transistors. One is a transistor with a Schottky diode in parallel with the collector junction. Due to the large leakage current at high temperature, its ts-Vcesat trade-off relationship is the worst, and it is currently less used. The other is a lateral PNP clamped transistor, whose structure is shown in Figure 6. It has a small leakage at high temperature, can obtain a better trade-off relationship, and the current amplification factor remains basically unchanged. It has been increasingly used. For example, the product 3DD13003A developed by Jilin Huawei Electronics Co., Ltd. adopts this structure.

Figure 6 Structure of lateral PNP clamp transistor

Table 3 shows the average efficiency of the system using APT13003E and 3DD13003A irradiated with 10 kGy electrons in the AP3765 series charger at input voltages of 85 V and 230 V, with output load currents of 0.15 A, 0.30 A, 0.45 A, and 0.60 A, respectively. As can be seen from Table 3, the efficiency of APT13003E after 10 kGy electron irradiation is basically the same as that of 3DD13003A.

Table 3 Comparison of the efficiency of the following three switching transistor systems used in the AP3765 charger

The electron irradiation process is simple and low-cost. After irradiation, the reverse breakdown voltage of the switching transistor will increase, the reliability of the switching transistor will increase, and the characteristic frequency will remain basically unchanged. Its disadvantage is that the current amplification factor is reduced, and it may not be able to conduct normally in high-power applications. Therefore, it is mainly used in small and medium-power switching circuits. The lateral PNP clamped switching transistor has basically no effect on the current amplification factor. Since a pn junction is added to the side, the transistor area and junction capacitance will increase, reducing the characteristic frequency of the transistor, but it cannot increase the reverse breakdown voltage. It can be used in bipolar digital circuits and small and medium-power switching circuits.

5 Conclusion

At higher AC input voltages, the system efficiency does not improve due to excessive conduction losses caused by charging of the transformer parasitic capacitance and the existence of the collector current "tail" during the turn-off phase. Since electron irradiation increases conduction losses and on-state losses, only by using a suitable electron irradiation dose can the system efficiency be maximized. The efficiency of a switching power supply system using a switching transistor with a suitable electron irradiation dose is basically the same as that of a switching power supply system using a lateral PNP clamp transistor.