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Posted on 01 February 2019

Design Software for Non Isolated SMPS

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Using the new VIPer12A and VIPer22A software

To supply the large number of electronics components required for home appliances, different power supply topologies can be used. We focus our attention on small appliances such as washing machines and white goods using non isolated SMPS.

By Giacomo Mercadante, STMicroelectronics

 

The worldwide home appliance market will increase to about 500 million units by 2010, greatly increasing the number of electronics components inside. New appliances have to be more and more user friendly and flexible. To cope with the constant market change driven by consumer demand, electronic designers are continuously researching new advanced solutions.

To supply the large number of electronics components required for home appliances, different power supply topologies can be used. If we focus our attention on small appliances such as washing machines and white goods, due to the physical layout of these applications, non isolated power supplies can be used.

Switch Mode Power Supply is a very common approach to meet the power needs of today’s appliances. In the case of low power applications, a non isolated buck converter (or buck-boost) is the best solution.

STMicroelectronics provides design software to simplify the design of non isolated switch mode power supplies. The user can easily develop an SMPS to control output power up to 17W. Using the VIPer12 and VIPer22, the user can build power supplies with a positive output voltage in buck topology or negative output voltage in buck-boost topology. The software, which has a very friendly GUI, calculates the main electrical and physical parameters, the total dissipated power, plots the main electrical waveforms of the converter and provides the Bill of Materials (BOM).

Software architecture

The user must first insert the input and output data: minimum input voltage, maximum input ripple voltage, input voltage line frequency, hold-up cycles (NH), output power (current and voltage) and output voltage ripple. Starting from the output power, the software will select a part number belonging to ST’s VIPer family and depending on the input data it will return the input bulk capacitor value CIN, using the following formula:

Formula 1

During the design, the user can change the part number suggested by the software.

A buck or buck-boost converter works in continuous mode (CCM) or in discontinuous mode (DCM). Which mode to use depends on different factors: the drop voltage between input-output, the switching frequency and the inductor value. Usually DCM is preferred to CCM because it guarantees better system stability, lower switching losses, smaller size and uses a less expensive inductor. Compared to CCM, the major drawback of DCM is the higher peak current, thus more stress on the devices. The software moves from DCM to CCM based on the following formula:

Formula 2

For a given output current, the operating mode switches from CCM to DCM if the inductance value decreases, the output power decreases or the input voltage increases.

Software schematic window (buck converter)

For given input and output data, the user selects the operating mode and the software automatically returns the inductance value. Conversely, the user can deselect the flag ‘automatic inductor calculation’, choose the inductor value, and the operating mode will be automatically selected depending on the other boundary conditions.

The function of the capacitor in the output stage of an SMPS is to store energy. The output capacitor value of the buck converter is usually selected to limit the output voltage ripple versus the needed value. This is not an ideal capacitor, but it has a series inductance (ESL: equivalent series inductance) and a series resistance (ESR: equivalent series resistance).

VIPer setting window

In order to evaluate the output voltage ripple, all series components should be considered. If the switching frequency is lower than approximately 300kHz, the ESL can be neglected and the voltage ripple can be attributed to the capacitor and the ESR. If the user selects the power output stage, the frequency and the inductor value, the software (through appropriate equations) returns the output capacitor value, the ESR value and the RMS ripple current. The calculated ripple can be checked by the output stage software window. If needed, the user can adjust the ripple to the needed value and the software will return the new capacitor and ESR values.

Waveforms plot window

The supply voltage of the VIPerx2 (VDD) can easily be obtained by connecting a diode and a capacitor (CVDD) to the output voltage. The diode voltage rating depends on the input voltage, while the current rating is not an issue. The capacitor value has an impact on the start-up time. Moreover, after a short circuit event, the VDD voltage falls and if the off level of the converter (VDDON - VDDhyst) is reached, a new start-up phase will be enabled. Therefore the time during which the switch is properly supplied changes according to the CVDD capacitor value.

The software returns the CVDD through the following equation:

Formula 3

The VIPer setting window (see Figure 3) provides the calculated value of power losses (PDISS) and the device temperature (TDEV). The total power losses are given by the sum of conduction losses, switching losses, and bias losses. Given the VIPer junction-case resistance (RJC), the device temperature can be calculated by the following equation:

Formula 4

In the VIPer selection window, if needed, the user can adjust the ambient temperature and the case-ambient resistance.

An example of a buck converter schematic with a single output is shown in Figure 2. The power circuit consists of the input rectifier, a bulk capacitor, the VIPer12A power switch, a free-wheeling diode and the output LC filter. A detailed project BOM can be printed in a text version and the schematic can be saved in JPEG format. Moreover, the mains power supply waveforms (drain current, drain-source voltage and power dissipation) can be displayed and printed (see Figure 4). The software comes with documentation and a detailed guide, helping the designer in the step-by-step design of an SMPS using the VIPer12 andVIPer22.

 

 

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