How to understand the power consumption of transistors

bigbat

How to understand the power consumption of transistors [Copy link]

 

This is the parameter of a Darlington transistor, Pc=1.5W, the collector power consumption is 1.5W, my understanding is: the power consumption of this component is 1.5W when it is working, that is to say; when it is working at full capacity, the current is 8A, 1.5/8=0.125V, that is, the voltage drop of the tube is 0.125V. Or how to understand the parameters of this tube?

Question 2: My workload is a solenoid valve with a 14W coil and a working voltage of 12V 14W/12V=1.17A. Can I just use any 2A transistor? There is no need for a Darlington at all.

Question 3: For this application, is it better to use transistors or MOS? Which one is more reliable and stable?

bigbat

The power consumption of a transistor is 1.5W. Does it mean that it is always 1.5W no matter how large the current is when it is working?

maychang

bigbat posted on 2022-6-25 09:38 The power consumption of a transistor is 1.5W. Does it mean that it is always 1.5W no matter how large the current is when it is working?

It says on the picture: MAXMUM RATINGS, which obviously does not mean "1.5W regardless of the current during operation". Just as the maximum current in the picture is 8A, it obviously does not mean that the current is 8A regardless of the state of the transistor. 1.5W, 8A, 100V, etc. are all maximum allowable values, which must not be exceeded. If exceeded, the tube may be damaged.

maychang

bigbat posted on 2022-6-25 09:38 The power consumption of a transistor is 1.5W. Does it mean that it is always 1.5W no matter how large the current is when it is working?

"Does it mean that no matter how much current is used during operation, it is always 1.5W?"

It is enough to consider one special case: what is the actual power consumption when the current is zero?

bigbat

maychang posted on 2022-6-25 11:32 "Does it mean that no matter how much the current is during operation, it is always 1.5W. ?" It is enough to consider one special case: what is the actual power consumption when the current is zero?

The transistor is connected in series with the load in the circuit. The current between CE is zero, so the power consumption of the transistor should be zero. The current of BE is not large, about a few milliamperes.

maychang

bigbat posted on 2022-6-25 13:46 The transistor is connected in series with the load in the circuit. The current between CE is zero, so the power consumption of the transistor should be zero. The current of BE is not large, about a few milliamperes.

The current between the base and the emitter multiplied by the emitter junction voltage drop is the emitter junction power consumption. The emitter junction voltage drop is usually around 0.7V, and the current is several milliamperes, so the emitter junction power consumption is several milliwatts.

maychang

bigbat posted on 2022-6-25 09:38 The power consumption of a transistor is 1.5W. Does it mean that it is always 1.5W no matter how large the current is when it is working?

"The power consumption of a transistor is 1.5W. Does that mean it is always 1.5W no matter how much current it uses during operation?"

The instantaneous power of any component is the voltage across its two ends multiplied by the current passing through it. Resistors, capacitors, inductors, diodes, transistors... are all no exception.

bigbat

maychang posted on 2022-6-25 14:10 "The power consumption of a transistor is 1.5W. Does it mean that it is 1.5W regardless of the current when it is working? " For any component, its instantaneous power is the voltage across its two ends multiplied by...

My question is that I don't know how to understand the power of 1.5W. Transistors and diodes have a fixed voltage drop. For example, if the transistor is 0.7V, 1.5W/0.7V=2.14A, then it is impossible for the transistor to have a current of 8A flowing through it.

maychang

bigbat posted on 2022-6-25 14:17 My question is that I don’t know how to understand the power of 1.5W. Transistors and diodes have a fixed voltage drop. For example, a transistor has 0.7V, and 1.5W/0.7V=2. ...

"My problem is that I don't know how to understand the power of 1.5W."

I have already said on the 3rd floor: What is written on the picture is: MAXMUM RATINGS, which means the maximum level. Vcbo is 100V, which means that if Vcbo exceeds 100V, the tube may be damaged. Vebo is 5V, which means that if Vebo exceeds 5V, it may be damaged. Pc is 1.5W, which means that if Pc exceeds 1.5W, it may be damaged. This is how the power of 1.5W is understood.

maychang

bigbat posted on 2022-6-25 14:17 My question is that I don’t know how to understand the power of 1.5W. Transistors and diodes have a fixed voltage drop. For example, a transistor has 0.7V, and 1.5W/0.7V=2. ...

"Both transistors and diodes have a fixed voltage drop, such as 0.7V for transistors."

There is no such "fixed voltage drop" for transistors.

bigbat

maychang posted on 2022-6-25 14:35 "Both transistors and diodes have a fixed voltage drop, such as 0.7V for transistors." Transistors do not have such a "fixed voltage drop."

Isn't the BE of a transistor a "diode"? I always thought that there must be a 0.7V voltage drop between BE, so there will be a fixed resistance between CE. Is this understanding correct? I am confused again.

maychang

bigbat posted on 2022-6-25 14:46 Isn't the BE of a triode a "diode"? I always thought that there must be a 0.7V voltage drop between BE, so there will be a...

"Isn't the BE of a triode a "diode"? I always thought there must be a 0.7V voltage drop between BE."

The previous sentence is generally correct, there is indeed a PN junction between BE.

The latter sentence is obviously wrong. If there is no current between BE (for example, the transistor has not been opened), this 0.7V voltage drop will definitely not exist. Even if there is current flowing through the emitter junction, the voltage drop between BE will change with the current, but the change is relatively small (the change is less than proportional).

maychang

bigbat posted on 2022-6-25 14:46 Isn't the BE of a triode a "diode"? I always thought that there must be a 0.7V voltage drop between BE, so there will be a...

"Then there will be a fixed resistance between CE."

This is a big mistake.

There is indeed resistance between CE, which is the output resistance of the bipolar transistor. However, this resistance value is not fixed, but changes with the voltage between the collector and emitter (the change is relatively small) and with the base current (the change is relatively large, especially when the collector-emitter voltage is very small). This is the slightly variable equivalent resistance of the transistor collector. It is definitely not fixed.

maychang

bigbat posted on 2022-6-25 14:46 Isn't the BE of a triode a "diode"? I always thought that there must be a 0.7V voltage drop between BE, so there will be a...

The ratio of the transistor collector voltage to the collector current is a DC resistance. This resistance value also changes with the transistor base current. The reason why the transistor has an amplification effect is that the collector current changes with the base current under certain conditions.

maychang

bigbat posted on 2022-6-25 14:46 Isn't the BE of a triode a "diode"? I always thought that there must be a 0.7V voltage drop between BE, so there will be a...

To understand this, it is best to look closely at the family of transistor output characteristic curves. A lot can be seen from these curves.

bigbat

maychang posted on 2022-6-25 15:04 "Isn't the BE of a triode a "diode"? I always thought that there must be a 0.7V voltage drop between BE." The previous sentence is generally...

The structure between the transistor CE is a meat-stuffed bun, but the thickness of the meat layer is controlled by the current of BE. If you want to leak oil through the meat layer, there must be a minimum pressure ground. Therefore, there should be a minimum voltage to overcome the minimum voltage drop, just like a diode.

bigbat

bigbat posted on 2022-6-25 15:19 The structure between the transistor CE is a meat sandwich, but the thickness of the meat layer is controlled by the current of BE. If you want to leak oil through the meat layer, there must be a minimum pressure...

I will do an experiment to see if the voltage drop of CE is constant when the transistor is saturated.

maychang

bigbat posted on 2022-6-25 15:19 The structure between the transistor CE is a meat sandwich, but the thickness of the meat layer is controlled by the current of BE. If you want to leak oil through the meat layer, there must be a minimum pressure...

The voltage drop between the collector and emitter of a transistor can be smaller than the voltage drop caused by the base current on the emitter junction. For an NPN tube, the collector potential is lower than the base potential at this time, that is, the collector junction is in the forward direction. This is completely different from a diode.

maychang

bigbat posted on 2022-6-25 15:27 I will do an experiment to see if the CE voltage drop of the transistor is fixed when it is saturated

Is the CE voltage drop of the transistor fixed when it is saturated?

Of course not. This voltage drop changes with the collector current. But it is not proportional. If it is proportional, it will become a resistor.

呜呼哀哉

The parameter you are looking at refers to the maximum collector dissipation power, which is the heat dissipation capacity from the junction temperature to the environment determined by the package. It is not what you understand as calculating the current. In addition, in addition to the voltage and current limit parameters, it is best to refer to the SOA curve when looking at the operating current. Your saturation voltage drop should still be referenced. Which curve is more suitable for VCE voltage drop and IC current?