Power switch

Standard for control cables Power switching equipment Panel boards... 

As one can notice, the boost-mode converter has the same parts as the forward-mode converter, but they have been rearranged. This new arrangement causes the converter to operate in a completely different fashion than the forward-mode converter. This time, when the power switch is turned on, a current loop is created that only includes the inductor, the power switch, and the input voltage source. The diode is reverse-biased during this period. The inductor s current waveform (Figure 3-4) is also a positive linear ramp and is described by... 

What is the peak current through the power switches ... 

Each topology has predictable voltage and current stresses for the power switches and rectifiers. These estimates have about a 90 percent confidence factor. Selecting the power devices at this stage in the design cycle can save precious time later in the program by not having to wait for parts. Table 3-2 contains equations that may be conservative in nature, but will work in the application. 

Bipolar Power Switch MOSFET Power Switch Rectifier(s) ... 

When the topology has multiple power switches, then multiply the Pioss(ckt) by... 

Topology Power Switch Type Bipolar MOS Overall Estimated Efficiency (%) Estimated Percent of Total Loss (P(%)) ... 

Power Switch and Drive (%) Output Rectifier (%) Magnetics (%) Miscellaneous (%)... 

A dc restoration circuit is needed following the output coupling capacitor to make the drive voltage referenced to the power switch s common. The supply voltage of the driver should be well bypassed so that its voltage does not droop during the drive pulse. 

Remember that a forward-mode transformer reflects the impedance from one side to the other. This means that if drivers are single-ended on the primary side (i.e., active turn-on, passive turn-off) the power switch will still have a slow turn-off. If totem-pole outputs are driving the primary are used, the power switch s response will be fast. 

The main purpose behind the power switch section is to convert the dc input voltage to a pulsewidth modulated ac voltage. The following stages can use a transformer to step-up or step-down the ac waveform, and finally the output stage converts the ac into the dc output(s). To accomplish the dc-to-ac conversion, the power switch operates only in the saturated and cutoff states. This makes the losses as low as possible. 

There are two major types of power switches used today the bipolar power transistor (BJT) and the power MOSFET. The IGBT (integrated gate bipolar transistor) is used in the higher power industrial applications, such as 1 kW power supplies and electronic motor drives. The IBGT has a slower turn-off than does the MOSFET, so it is typically used for switching frequencies of less than 20 kHz. 

The power MOSFET is the most common choice as a power switch. Its cost and saturation loss are comparable to the bipolar transistor in most applications and it switches five to ten times faster. It is also easier to use in a design. 

One last form of voltage-mode eontrol is very rudimentary. It eould be ealled hysteretic voltage-mode. In this form of eontrol, a fixed frequency oscillator is gated ON only when the output voltage has fallen to below a limit dictated by the voltage feedback loop. It is sometimes called hiccup-mode because the power switch occasionally bursts on and then returns to a constant off state. 

Figure 3-58 Methods of switching power buses (a) dual switching power supply with enable (b) a dc power switch.

The power switch and rectifier ac-current loops contain very high trapezoidal current waveforms typical in PWM switching power supplies. These waveforms are rich in harmonics which extend far above the basic switching frequency. These ac currents can have peak amplitudes two to five times that of the... 

These ac current loops should be routed before any other traces in the power supply. The three major components that make up each loop the filter capacitor, the power switch or rectifier, and the inductor or transformer must be located adjacent to one another. The components must also be oriented such that the current path between them is as short as possible. A good example of a layout of the power section of a buck (or step-down) converter can be seen in Figure 3-60. 

There is one node within each switching power supply that has the highest ac voltage compared to the others. This node is the ac node found at the drain (or collector) of the power switch. In nonisolated dc/dc converters, this node is also connected to the inductor and catch (or output) rectifier. In transformer-isolated topologies, there are as many ac nodes as there are windings on the transformer. Electrically, they still represent a common node, only reflected through the transformer. Special attention must be paid to each ac node separately. 

This node(s) presents a different problem. Its AC voltage can be easily capac-itively coupled into any adjacent traces on different metal layers, as well as radiate EMI. Unfortunately, it is generally the trace that must also act as a heatsink for both the power switch and the rectifiers, especially in surface mount power supplies. Electrically, the trace wants to be as small as possible, but thermally, it wants to be large. There is one good compromise in the surface mount designs, and that is to make the top PCB island identical to the bottom PCB island and connect them with numerous vias (or thru-hole connections). This can be seen in Figure 3-62. 

Power switch. The power switeh is going to be a P-ehannel power MOSFET. The maximum input voltage is 18VDC. Therefore, a Loss rating of -I-30VDC or higher will be satisfaetory. The peak eurrent is 2.8 A. It is also desired to keep the heat dissipation less than 1 watt so the estimated RDS(on) should be less than ... 

This capacitor experiences the same current waveform at the power switch, which is a trapezoid with an initial current of about 1A rising to 2.8 A with very sharp edges. This capacitor has much more rigorous operating conditions than the output filter capacitor. I will estimate the RMS value of the trapezoidal current waveform as a piecewise superposition of two waveforms, a rectangular 1A peak waveform and a triangular waveform with a 1.8 A peak. This yields an estimated RMS value of 1.1 A. The value of the capacitor is then calculated as ... 

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