Tag Archives: Transistors

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How Does Switching Affect Semiconductors?

Even though ICs rule the world of electronics, the transistor does all the work. Within each IC are millions upon millions of transistors perpetually switching on and off so that the IC can carry out its intended functions. Even if one of the multitudes of transistors were to stop switching, the IC could lose part or all of its functionality.

Circuits handling digital signals most often use transistors to switch from a high state to a low state and vice versa. It is usual to call a circuit point as being in a high state if the voltage at that point is close to the supply voltage. If the circuit point is closer to the ground or zero voltage, we generally call it as being at a low state. The time taken for the transistor to switch from a high to a low state or vice versa is its switching rate. While the transistor does not expend much energy when at either the low or the high state, the same cannot be said for the time when it is actually switching.

Under ideal conditions, a transistor should switch instantaneously. That means the transistor should take zero seconds to change its state. However, ideal conditions do not happen in reality and the transistor takes a finite time, however small, to actually switch over.

Transistors are made of semiconductor material and each junction has a finite capacitance and resistance. Junction capacitances store energy and the combination of resistance and capacitance acts to slow down switching – the capacitance must fill up or empty itself before the transistor can flip. The rate at which the capacitance fills up or empties itself depends on the junction resistance.

The situation gets worse as the switching frequency goes up. As the transistor is driven to toggle faster and faster, the junction capacitance may not get enough time to discharge or charge up fully. That defines the maximum switching rate the transistor can achieve.

Semiconductor manufacturers use various methods to reduce junction capacitances and resistances to induce these special semiconductors switch faster. Although modern semiconductors (transistors and diodes) are capable of switching at MHz or GHz scales, the cumulative effect of the tiny switching losses add up to increase the junction temperature.

Power is the product of voltage and current. When a semiconductor is in a high state, although the voltage is high, the current is negligible and consequently, the power drawn from the supply is negligible. When the semiconductor is a low state, its voltage is close to the ground level and the product of current and voltage is again negligible.

However, during switching, when the voltage is somewhere in-between the supply and ground levels, the current drawn also increases. That makes the product of voltage and current have a significant value and the semiconductor generates heat because of the power consumption. With higher frequencies, this happens more frequently and the heat accumulates to produce higher junction temperature.

If the natural process of heat dissipation can remove the accumulated heat, the semiconductor soon reaches a steady temperature. Else, heatsinks and or forced cooling methods are necessary to remove the heat accumulated.

How RTPs Help To Save Expensive PCBs from Thermal Runaway PowerFETs

powerfetAlthough powerFETs or power Field Effect Transistors are very robust devices used in the automotive industry – they have their limitations. In the automotive environment, powerFETs go through the tortures of extreme temperature variations together with severe thermo-mechanical stresses. They face noisy short circuits, high arcing, intermittent shorts as well as inductive loads. These shocks can fatigue the device over time, and it can fail in a short, an open or resistive mode.

For example, if the maximum operating voltage of a powerFET is exceeded, failure happens very quickly. The powerFET goes into an avalanche breakdown once the voltage rating goes beyond the maximum allowed. If the energy within the transient overvoltage is more than the rated avalanche energy level, the device will start to fail resulting in generation of smoke, flame or it may even be de-soldered.

In some cases, the powerFET while failing may generate precarious temperatures through I2R heating. This may cause a thermal runaway for the device, but the resulting current may not be large enough to cause failure of a standard fuse protecting the powerFET. This mode of failure is of particular concern, for not only the powerFET, but also for the PCB or the Printed Circuit Board. A power of as little as 10 Watts may generate localized hot spots of above 180°C, which can damage the glass PCB’s epoxy structure leading to a thermal event.

Tyco Electronics has developed a Reflowable Thermal Protection or RTP, which is a reliable and robust surface mount thermal protector to prevent thermal damage on PCBs caused by failing power electronics. This is a secondary thermal protection device, which can replace several components such as redundant powerFETs, heavy heat sinks and relays currently used for such protection in the automotive designs.

To work effectively, the RTP device has to be placed in series and on the power line, very close to the FET. This allows the device to track the temperature of the FET and disrupt the current by opening the circuit before the thermal runaway condition generates a thermally destructive condition on the PCB. Under normal conditions, the RTP device has a low resistance, typically about 0.6mOhm.

Whenever the RTP device detects the generation of unsafe temperatures because of the failure of a power component or any other board defect, it interrupts the current and prevents a thermal runaway condition that could lead to critical damage. An RTP200 device typically opens (high resistance condition) at 200°C, which is a temperature above the normal operating temperature, but below the Lead-free solder reflow temperature.

It may seem like a paradox that the RTP device operates at 200°C but can withstand Lead-free soldering temperatures of 220°C. This is because the RTP is not in an active state unless it has been armed by passing a specific current through it for a specified amount of time. Before it is armed, the RTP can withstand three Lead-free solder reflow steps before it operates. The electronic arming procedure is one-time only and can be implemented to occur automatically or during system testing.