Industrial Electronics –I(Power Transistor)

Study of Losses in Power Semiconductor Devices – Calculation of Loss in Power BJT: -

Power semiconductor devices form the backbone of modern power electronics. They are used in converters, inverters, motor drives, power supplies, and almost every application where electrical energy needs to be efficiently controlled and converted. Among these devices, the Power BJT (Bipolar Junction Transistor) once played a dominant role before the arrival of MOSFETs and IGBTs. Even today, understanding its operation and losses is important for students and engineers because the same concepts apply, in modified form, to other devices.

In this blog, we will study losses in power semiconductor devices, with a focus on power BJTs. We will cover conduction losses, switching losses, how to calculate them, and why they are critical for device performance and thermal management.

1. Introduction to Losses in Power Semiconductor Devices

Every power semiconductor, whether it is a BJT, MOSFET, IGBT, or thyristor, dissipates some amount of power when operating. This power loss converts into heat, which must be managed through heat sinks, cooling systems, or thermal designs. The losses are generally divided into two broad categories:

A. Conduction Losses—when the device is carrying current.
B. Switching Losses—when the device transitions between ON and OFF states.

Additionally, there may be leakage losses, drive circuit losses, and snubber circuit losses, but the major contributors are conduction and switching.

2. Power BJT—A Quick Refresher

A bipolar junction transistor is a three-terminal device (emitter, base, and collector) that works on the principle of current amplification. In power applications, it is mostly used as a switch, not as a linear amplifier.

• When base current is provided, the device enters saturation and conducts large collector current with a small collector-emitter voltage drop (VCE(sat)).
• When base current is removed, the device turns OFF, but not instantly—it has storage time and fall time, which contribute to switching losses.

ON state (VCE(sat)) and OFF state (IC=0).

3. Types of Losses in Power BJTs

(A) Conduction Loss

When the BJT is in the ON state, it has a voltage drop across the collector-emitter terminals, denoted as VCE(sat). The conduction loss is given by:

Pcond = VCE(sat) × IC × D

Where:

• VCE(sat) = Collector-Emitter saturation voltage
• IC = Collector current
• D = Duty cycle

(B) Switching Loss

When the transistor switches from ON to OFF or vice versa, there is a short time where both current and voltage are significant. This leads to switching losses.

The total switching loss per cycle is

The average switching power loss is:

Where:

• VCE Voltage across the transistor during switching
• IC = Collector current
• ton = Turn-on time (delay + rise)
• toff = Turn-off time (storage + fall)

The average switching power loss is:

Psw = Esw × fs

Where fs is the switching frequency.

(C) Total Loss

The total power loss in a power BJT is

Ptotal = Pcond + Psw

Timing diagram showing td, tr, ts, and tf (delay, rise, storage, and fall).

4. Step-by-Step Calculation of Loss in a Power BJT

Example Parameters:

• VCE(sat) = 1.5 V
• IC = 20 A
• Duty cycle D = 0.4
• Switching voltage VCE = 100 V
• Switching times: ton = 0.8 μs, toff = 1.2 μs
• Switching frequency fs = 20 kHz

So, the BJT will dissipate 52 watts of heat in this case. This must be managed with a suitable heat sink and possibly forced cooling.

5. Importance of Loss Calculation

• 1. Thermal Design—Junction temperature must be kept below limits (typically 150–175°C).
• 2. Efficiency – Losses directly reduce system efficiency.
• 3. Reliability – Overheating causes permanent damage.
• 4. Device Selection – Helps in choosing between BJT, MOSFET, or IGBT depending on switching frequency and load.

6. Comparison with Other Devices

- MOSFETs have lower switching losses at high frequency because they are majority-carrier devices.
- IGBTs combine the low conduction loss of BJTs with the fast switching of MOSFETs.
- Thyristors are still used for very high power, low-frequency applications.

7. Conclusion

The study of losses in power semiconductor devices is essential for designing efficient and reliable power electronic systems. In the case of power BJTs, the two main contributors are conduction losses (due to VCE(sat)) and switching losses (due to finite turn-on and turn-off times). By performing proper loss calculations, engineers can determine how much heat will be generated, how large the heat sink should be, and whether the device is suitable for the intended application. Although power BJTs are now less commonly used compared to MOSFETs and IGBTs, the fundamental principles of loss analysis remain highly relevant.

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