---
title: "Power transistor V-I and static switch"
subject: "Power Electronics"
chapter: 1
key: "semester_7/power_electronics:power-transistor-vi-static-switch"
source: https://www.bctengineer.com/q/semester_7/power_electronics/power-transistor-vi-static-switch
site: BCT Engineering
---

# Power transistor V-I and static switch

Power Electronics · Chapter 1: Characteristics and specification of power electronic devices

- Asked in: 2079 Baishakh, 4 marks; 2072 Chaitra, 8 marks (asked 2 times)
- Video: [Power BJT (Basics, Symbol, Structure, Working, Modes, VI Characteristics & Applications) Explained](https://www.youtube.com/watch?v=fndnvTAEP0I) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_electronics/power-transistor-vi-static-switch
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1

## Question

Explain the V-I characteristics of power transistor and illustrate how it can be used as a static switch.

## Short answer

The V-I characteristic has cut-off, active, and saturation regions. As a static switch, the transistor is OFF in cut-off with negligible current and ON in saturation with VCE(sat) about 0.2 to 1 V; base current is overdriven, with ODF 1.5 to 3, while the active region is avoided for low loss.

## Answer

A **power transistor (BJT)** is a three-layer, two-junction device (NPN or PNP) with a vertical structure suited to high voltage and current. Its V-I (output) characteristic is the graph of collector current $i_C$ against collector-emitter voltage $v_{CE}$ for different base currents $I_B$.

#### V-I characteristics

```text
   iC
    ^ saturation
    | |     IB4
    | |----------------------
    | |     IB3
    | |----------------------  active
    | |     IB2               region
    | |----------------------
    | |     IB1
    | |----------------------
    |/_______________________  IB = 0
    +----------------------------> vCE
            cut-off region
```

- **Cut-off region:** $I_B = 0$; both junctions reverse biased; only a small leakage current $I_{CEO}$ flows. The transistor is OFF.
- **Active region:** base-emitter junction forward biased, collector-base reverse biased; $i_C = \beta I_B$. The device acts as an amplifier, but the power loss $v_{CE} i_C$ is large, so power circuits avoid this region.
- **Saturation region:** both junctions forward biased. $i_C$ is set by the external load and $v_{CE(sat)}$ is very small (0.2 to 1 V). The transistor is fully ON.
- **Quasi-saturation:** in power BJTs, the lightly doped collector drift region gives a region between active and hard saturation; operating here keeps storage time small.
- A **primary breakdown** ($BV_{CEO}$) and a **second breakdown** limit the safe operating area.

#### Use as a static switch

```text
                 +Vcc
                  |
                 RL (load)
                  |
          Rb     C|  iC
   vB o--/\/\/---B|/
                  |\  NPN
                 E|
                  |
                 GND
```

- **OFF state (cut-off):** when $v_B \le 0$, $I_B = 0$, both junctions are reverse biased and $I_C \approx 0$ (only leakage). The transistor acts as an open switch and the full supply voltage $V_{CC}$ appears across C-E.
- **ON state (saturation):** when enough base current is supplied, both junctions are forward biased. $V_{CE(sat)}$ is only about 0.2 to 1 V, so the transistor acts as a closed switch and $I_C = \frac{V_{CC} - V_{CE(sat)}}{R_L}$.
- To be sure of saturation, the base current is made larger than the minimum value:
$$
I_B > I_{BS} = \frac{I_{CS}}{\beta_{min}}, \qquad \text{ODF} = \frac{I_B}{I_{BS}}
$$
where ODF (overdrive factor) is usually 1.5 to 3 (too much overdrive increases storage time).
- The active region is avoided because both $V_{CE}$ and $I_C$ are large there, giving high power loss $V_{CE} I_C$. In the switching mode the loss is small: in cut-off the current is nearly zero, and in saturation the voltage is nearly zero.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the active region of a power BJT, which junction conditions and relation hold?
   A. Both junctions forward biased, iC set by load
   B. Both junctions reverse biased, iC = ICEO
   C. B-E forward, C-B reverse biased, iC = beta IB
   D. B-E reverse, C-B forward biased, iC = beta IB
   Answer: C. In the active region the base-emitter junction is forward biased and collector-base is reverse biased, so iC = beta x IB. (Key point: Active region)

2. In the saturation region, what determines the collector current iC?
   A. beta times IB
   B. The external load, with VCE(sat) very small (0.2 to 1 V)
   C. The leakage current ICEO
   D. The breakdown voltage BVCEO
   Answer: B. In saturation both junctions are forward biased; iC is set by the external circuit and VCE(sat) is small, so the transistor is fully ON. (Key point: Saturation region)

3. What happens in the cut-off region used for the OFF state of the switch?
   A. IB = 0, both junctions reverse biased, only leakage ICEO flows
   B. IB is large and VCE is near zero
   C. The collector current is beta times IB
   D. Second breakdown occurs
   Answer: A. With IB = 0 both junctions are reverse biased, so only a small leakage current flows and the switch is open. (Key point: Cut-off region)

4. How is the minimum base current for saturation IBS found?
   A. IBS = ICS x beta_min
   B. IBS = ICS / beta_min
   C. IBS = VCC / RL
   D. IBS = VCE(sat) / beta_min
   Answer: B. IBS = ICS/beta_min, and the actual IB is made larger by the overdrive factor (1.5 to 3). (Key point: Minimum base current)

## Related in Power Electronics

- [Transistor as static switch and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Atransistor-static-switch-base-drive) — Power Electronics, chapter 1, asked 4×
- [Power transistor switch and opto-coupler isolation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apower-transistor-switch-opto) — Power Electronics, chapter 1
- [Transistor switch with opto-coupler base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Atransistor-switch-opto-base-signal) — Power Electronics, chapter 1
- [BJT as a switch in power circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-as-power-switch) — Power Electronics, chapter 1, asked 2×
- [Power transistor switching and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apower-transistor-switching-base-drive) — Power Electronics, chapter 1

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