(Part 1 looked at linear and switching regulators, along with loss analysis and efficiency.)
Switching topologies, by their very nature, are more efficient than their linear counterparts. These topologies come in various forms and their components play a significant role in the overall efficiency of the circuit. This article will illustrate the half-bridge and inverter topologies, the end applications, and how their components and component parameters affect overall efficiency.
An inverter is three half-bridge circuits in parallel. The configuration is used to provide a three-phase output such as what is required to drive an AC induction motor or DC Brushless motor.
Figure 8: Typical inverter module used in motor drives
Depending on the voltage and current level of the application either MOSFETs or IGBTs can be used. Figure 8 shows a three phase module schematic of an inverter that is intended for 500W to 3KW applications. In particular appliance motor drives.
Inverters, in general, are used to converter DC power into AC power. This application spans battery-powered inverters like the single-phase DC/AC 120W devices that can be purchased aftermarket for cars, to the large Uninterrupted Power Supplies (UPS) used for back-up power in industrial installations.
Efficiency of inverters is usually greater than 95%. Calculating the efficiency of an inverter requires an understanding of the load. Motor loads can be modeled as a series inductor and resistor. The following calculations are based on sinusoidal modulation.
Pout can be calculated as follows (Equation 6).
Pout can also be calculated by (Equation 7):
The PF can be calculated using the load impedance (Equation 8):
Vrms is the effective RMS voltage across the load. The actual voltage across the load is a PWM waveform that is similar to Figure 9.
Figure 9: Inverter PWM voltage across the LR load
The effective voltage across the load from Figure 9 is the following (Equation 9).
Efficiency is calculated using the losses in the inverter applied to Equation 4.
Unlike the Sync-Buck circuit, the duty is not fixed. A fixed duty cycle lends itself to a straightforward switch power loss calculation. When the duty cycle is sinusoidal in nature, there is no straightforward way to calculate the power dissipation.
One approach is to analyze one phase of the inverter using Spice, Figure 10.
Figure 10: Spice Model of One Phase of an Inverter
A ramp generator is modulated by a sine wave generator, in this case, with a 0.9 modulation index. A dead time is inserted to prevent shoot through which, as with a real circuit, will add additional power loss and add to EMI noise. E1, E2, R2 and R3 model the IGBT gate driver. Bus represents rectified and filtered 220Vac. L2 models the load (a motor’s inductance and resistance). V1 is the virtual ground of a Y connected motor.
Figure 11: Simulated input and output current of one phase of an inverter
In this example, the load is R=14.137? and L=50mh (PF=0.6 at 60Hz), the switching frequency is 15kHz. The analysis, Figure 11, yields an input power of 292W and an output power 285W, which results in 97% efficiency. This can be compared to an online inverter analysis tool, Figure 12.
Figure 12: Online inverter analysis tool
Figure 13 shows results from the Power Loss Analysis tool:
Figure 13: Results from the online tool
It also shows the full three-phase inverter loss of 34W at 15kHz. It can be assumed that one phase of the inverter dissipates 1/3 of the power of the full inverter. Thus the one phase power dissipation is 11.3W. Assuming Vp=170V, m=0.9 and applying to Eq. 8 gives Vrms=108.2V. Irms=4.5A in this example, thus (Equation 10):
Using Equation 7 to calculate efficiency yields (Equation 11):
(Note: The IGBT chosen in the Spice model is similar to the IGBT used in the module simulated in the online tool.)
Thus there is a 1% difference in the calculation of the efficiency between the two methods.
As with all converters the various application parameters affect their efficiency. The online tool can show how switching frequency and module (IGBT) section affects efficiency.
Figure 14: Inverter power dissipation for
Iout=4.5A fsw=2KHz-20Khz for 3 modules/IGBTs
Figure 14 shows for Vbus=340V, Iout=4.5A, and PF=0.6, the power dissipation is a function of the module/IGBT and switching frequency.
Electronic switches (MOSFET & IGBT) allow for highly efficient power conversion. This is made possible by the simple process of averaging the PWM voltage to yield the desired output voltage. Ideally the switch dissipates power only when it is on which is only for a portion of the switching cycle.
This power dissipation is significantly lower than if the power were to be transferred using a linear regulating element. Efficiencies are from 85% to around 97%, depending on the application parameters such as the difference between the input and output voltage, the switching frequency, the number of power dissipating elements in the conversion and the switches selected for PWM.
There are other tradeoffs made in the design of PWM converts that were not discussed in the article, such as EMI, thermal considerations, layout, and packaging. PWM strategy is the standard method for power conversion, thus making it invaluable in today’s electronic products.
About the author
Dave Divins is a Senior Field Application Engineer at International Rectifier Corp (El Segundo, CA). David graduated from The City University of New York with a BEEE and Binghamton University with a MSEE. He worked at GE Aerospace as a Systems and Hardware Engineer on Avionic Flight Control and Jet and Turbo Fan Engine Controls. He also worked at Ford Electronic (AKA Visteon) as a Components Engineer and as an Applications Engineer for Synopsis on the Saber Simulator. For the past 11 years he has worked at International Rectifier as a Senior Field Applications Engineer.