Читать книгу Origin of Power Converters - Tsai-Fu Wu - Страница 17

The major concern of a power converter is its input–output conversion efficiency. In practice, there is no resistor allowed in a converter configuration. A qualified converter includes only ideal switch(es) and capacitor(s)/inductor(s). However, even with these components only, there might still exist loss during power transfer, such as the converters shown in Figure 1.5a and b. Figure 1.5a shows power transfer between two capacitors, and it is controlled by switch S₁. Assuming capacitor C₁ is associated with an initial voltage of V_o and C₂ is with zero voltage, and capacitance C₁ = C₂, it can be shown that there is an electrical energy loss, , which is half the initially stored energy in C₁. Moreover, when switch S₁ is turned on, an inrush current flows from C₁ to C₂ and through S₁ in almost no time, which may damage the components and cause EMI problems. For this type of circuit configuration, the only current limiter is the equivalent inductance and resistance of the components and the circuit path. It can be said that there is no control on the capacitor currents and voltages, and the voltages of both capacitors C₁ and C₂ will be always balanced at (1/2)V_o. This type of power converter configuration is classified as a non‐PWM converter.

Similarly, the conceptual inductor–inductor–switch configuration shown in Figure 1.5b has the same limitations. If, initially, inductor L₁ carries a current of I_o but there is no current in L₂, after turning on switch S₁, there will be an extremely high impulse voltage across the inductors and the switch, causing EMI problems and damage to the components. Again, there is half electrical energy loss if L₁ = L₂, and there is no control on the inductor voltages and currents at all. It is also a non‐PWM converter.

Figure 1.5 (a) Capacitor–capacitor–switch, (b) inductor–inductor–switch, and (c) capacitor–inductor–switch networks.

In summary, non‐PWM converters come out high inrush current or high impulse voltage, resulting in high EMI, as well as high component stress, and they could yield low conversion efficiency even with ideal components. In particular, under large initial voltage difference, the maximum electrical energy loss can be as high as 50%.

Other examples adopting the configuration shown in Figure 1.5a are shown in Figure 1.6. Figure 1.6a shows a two‐lift converter. When switches S₁ and S₂ are turned on, capacitor C₁ will charge C₂ directly. On the other hand, when switch S₃ and S₄ are turned on, capacitors C₁ and C₂ are connected in series to charge capacitor C₃ and lift the output voltage V_o to be twice the input voltage V_i. It can be seen that during capacitor charging, there is no current limiter, resulting in high inrush current. Figure 1.6b shows the KY converter. When switch S₂ is turned on, input voltage V_i will charge capacitor C₁ through diode D₁ but again without current limiter. When switch S₂ is turned off and S₁ is turned on, input voltage V_i together with capacitor voltage V_C will magnetize inductor L₁ through the output path. This path of power flow is with the current limiter of inductor L₁. Figure 1.6c shows a re‐lift converter. When switch S₁ is turned on, there are two capacitor charging paths without current limiter, V_i‐S₁‐D₂‐C₃‐D₃‐V_i and V_i‐S₁‐D₂₁‐C₁₂‐D₁₁‐V_i. When switch S₁ is turned off, the energy stored in capacitors C₃ and C₁₂ will be released to the output through the inductors and capacitors, which are the current limiters.

Figure 1.6 Non‐PWM converters: (a) two lift, (b) KY, and (c) re‐lift circuit.

With a non‐PWM converter, the processed power level is usually pretty low because of high inrush current or high pulse voltage. It can be used for supplying integrated circuits, which require low power consumption, of which the low current rating switches have high conduction resistance and act as current limiters. For high power processing, we need PWM power converters.