Power BJT

Working of Power BJT is discussed here.

1. Power BJT By Shaik Hedayath Basha Assistant Professor RMK College of Engineering and Technology 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 1

2. Recall BJT… BJT is Bipolar Junction Transistor, it has three terminals : Emitter, Base and Collector. The Base area smaller and is lightly doped than Collector and Emitter. The Collector area less than or equal to Emitter area. The Emitter area is highly doped compared to Base and Collector 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 2

3. Recall BJT… 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 3

4. RECALL BJT Char… Curve 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 4

5. Power bjt : The  power BJT  has three terminals Collector (C), Emitter (E) and Base (B). It has a vertically oriented four-layers structure. The vertical structure uses to increase the cross-sectional area. The power bipolar junction transistor (BJT) blocks a high voltage in the off state and high current carrying capacity in the on-state. The power handling capacity is very high. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 5

6. Power bjt construction 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 6

7. Doping profile It has four layers. The first layer is a heavily doped  emitter layer (n+).   The second layer is moderately doped the  base layer (p).   The third region is lightly doped  collector drift region (n-).   The last layer is a highly doped  collector region (n+). 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 7

8. Note 1: The drift layer (n-) increase the voltage blocking capacity of the transistor due to the low doping level. The width of this layer decides the breakdown voltage. The disadvantage of this layer is that the increase on state voltage drops and increase on state device resistance, which increases power loss. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 8

9. NOTE 2: The power handling capacity of the power transistor is very large. So, they have to dissipate power in the form of heat. Sometimes, heatsink uses to increase effective area and therefore increase power dissipation capacity.  the heatsink made from metal 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 9

10. Power Bjt 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 10

11. Power bjt : It is designed to effectively manage the power dissipation. To avoid Krik effect. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 11

12. Krik effect Krik Effect: The Krik effect occurs at high current densities and causes a dramatic increase in the transit time of a bipolar transistor. This effect is due to the charge density associated with the current density passing through the base collector region. As charge density exceed in the depletion region, the depletion region ceases to exist. Krik effect can be eliminated by increasing the collector doping. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 12

13. Important points of power bjt Power BJT have thick and low doped collector region, it results in large blocking voltage. It has extremely low doping densities, used to obtain blocking voltages as large as 3000 Volts. It has large Active area then normal BJT, this result in higher current capabilities. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 13

14. I-V characteristic: The I-V characteristic of Power BJT divides into four regions . Cut-off region Active region Quasi-saturation region Hard saturation region 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 14

15. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 15

16. 1. Cut-off region: The BE and CB both junctions are reverse bias. The base current I B =0 and collector current I C  is equal to the reverse leakage current I CEO . The region below the characteristic for I B =0 is cut-off region. In this region, BJT offers large resistance to the flow of current. Hence it is equivalent to an open circuit. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 16

17. 2. Active region The BE junction is forward bias and CB junction is reverse bias. The collector current IC increase slightly with an increase in the voltage V CE  if I B  is increased. The relation of IB and IC is, I C =β dc I B  is true in the active region. If BJT uses as an amplifier or as a series pass transistor in the voltage regulator, it operates in this region. The dynamic resistance in this region is large. The power dissipation is maximum 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 17

18. 3. Quasi – Saturation region Quasi-saturation region is between the hard saturation and active region. This region exists due to the lightly doped drift layer. When the BJT operates at high frequency, it is operated in this region. Both junctions are forward bias. The device offers low resistance compared to the active region. So, power loss is less. In this region, the device does not go into deep saturation. So, it can turn off quickly. Therefore, we can use for higher frequency applications 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 18

19. 4. Hard – saturation region The Power BJT push into the hard-saturation region from the quasi-saturation region by increasing the base current. This region is also known as deep saturation region. The resistance offers in this region is minimum. It is even less than the quasi-saturation region. So, when the BJT operates in this region, power dissipation is minimum. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 19

20. 4… The device acts as a closed switch when it operates in this region. But it needs more time to turn off. So, this region is suitable only for low-frequency switching application. In this region, both junctions are forward bias. The collector current is not proportional to the base current, I C  remains almost constant at I C(sat)  and independent from the value of base current. 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 20

21. Application of Power BJT: Switched Mode Power Supply (SMPS) Power Amplifier Relay and Drivers AC motor speed controller DC/AC inverter As series pass transistor in the regulated power supply The audio amplifier in the stereo system Power control circuit 30-04-2020 EC 8252 Electronic Devices, RMK College of Engineering and Technology, Chennai 21