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White Paper on AC/DC Dual Purpose Power Supply Application

 

 

Foreword

 

In the past, the purpose of using the AC power supply was to provide the voltage and frequency of the utility power in various countries and to simulate various phenomena of abnormal power quality. With the development of new energy sources, in addition to using AC power as input, communications and server power supplies can also directly input DC power (HVDC or LVDC) from UPS or energy storage devices to save power consumption from AC to DC conversion.

 

In addition, the DC mode of the AC power supply has a faster voltage change response time (compared to the DC power supply), this characteristics can meet the testing needs of vehicle-mounted electrical equipment, and this White Paper will comprehensively describe these applications.

 

The bounden duty of testing instruments: Exploring the unknown with the known

 

Test instruments are output-type instruments. The output provided has two purposes. The first purpose is to provide an output with a known error to stimulate (Stimulate) the input of the DUT so that the DUT can operate; the second purpose is to simulate possible fault phenomena by the input to the DUT to observe the fault tolerance of the DUT or to reproduce the fault for circuit debugging.

 

Why can't the functional test be done directly by utility power?

 

Take the charger specifications of a notebook computer as an example, the specifications of the input terminals are： 

  • Rated input voltage AC 100~240V
  • Operating voltage range AC 90~264V
  • Rated frequency 50/60Hz±3Hz

 

The utility power cannot provide the rated input voltage range of 100~240V for the global universal charger, and must also provide a stable voltage tolerance range of ±10%, so it must be able to provide an operating voltage of 90~264V as the input excitation source.

 

The autotransformer is cheaper than the AC power supply and can provide a voltage of 90~264V. Why can't the autotransformer replace the AC power supply?

 

There are five reasons why autotransformer cannot replace AC power：

 

  1. It has no electrical isolation function.
  2. In addition to controlling the variable factor in experiments or product verification, it is necessary to observe the unknown from the known. The voltage of the utility power itself is unstable. Even if you use an electricity meter to confirm that the output of the autotransformer is 90V, it is difficult to guarantee the next second the output of the autotransformer to become 88V due to the change of the neighbor's load. The experimental purpose cannot be achieved by using the unknown to observe the unknown.
  3. The autotransformer cannot provide the frequency range of rated frequency 50/60Hz±3Hz.
  4. The output impedance of the autotransformer will distort the output sine wave, causing the PFC (Power Factor Compensation) circuit of the power supply circuit to track a problematic waveform.
  5. It will produce 50Hz or 60Hz low-frequency electromagnetic radiation to interfere peripheral circuits.

 

Electricity quality related standards

 

ITI1 (CBEMA2 ) Information Technology Industry Committee (Computer and Business Equipment Manufacturers Association): Founded in 1916, the committee was formerly abbreviated as CBEMA2 in 1973, and changed to its current name ITI1 in 1994. The purpose of the committee is to encourage innovation. Almost all the giants in the information technology industry are members. This Paper will use the power quality curve updated by the ITI committee in 2000 as an explanatory case.

 

In addition to ITI, related standards for power quality include IEEE 1159 Recommended Practice for Monitoring Electric Power Quality; IEC 61000-4-11; IEC 61000-4-34; Semi47: Specifications for processing voltage sag immunity).

 

Figure 1 is the ITI Curve, the Voltage Tolerances envelope curve of this figure is suitable for single-phase 120V equipment. There are three areas in the figure, the first area is the No interruption in function region; the second area is the Prohibited region; the third region is the No damage region)

 

 

 

Figure 1: ITI Curve (this figure refers to the ITI application article and adds Chinese notes and SEMI47 specifications)

 

 

Design countermeasures for the power circuit： 

  • No interruption in function region: Hold-up capacitor; feedforward; feedback control; surge absorption.
  • Prohibited region: Overvoltage protection (OVP3 )
  • No damage region: low voltage protection (UVLO4 )

 

The power converter control mechanism includes: voltage feedback; current feedback; input voltage feedforward; input current feedforward; temperature feedforward; load change feedforward, etc.

 

Power quality issues of no interruption in function region

 

From the voltage tolerances envelope curve of the no interruption in function region of the ITI curve function, it can be subdivided into several areas. These areas have their ownpower quality issues and countermeasures. These subdivision areas include：

 

  1. ±10% steady-state allowable area: Take 100V as the rated value in Figure 1, and 90~110V as the steady-state allowable area. In this interval, the equipment must always be able to operate normally.
  2. Voltage sag: The causes of voltage sag include motor startup period, line failure or affected interference (short circuit, damage, lightning strike), power supply equipment failure, transient load changes, etc. Possible effects include illumination flickering, voltage-sensitive equipment tripping (such as electromagnetic contactor MC or electromagnetic relay), and equipment not functioning properly (programmable logic controller PLC, high intensity discharge lamp HID). Products that meet the Semi47 specification for voltage sag tolerances are better than ITI and IEC-61000-4-11 specifications. ITI's specification is that voltage sags to 70% of rated value must maintain normal operation of equipment for 0.5 seconds; and that voltage sags to 80% of rated value must maintain normal operation of equipment for 10 seconds. The Semi47 additionally requires that voltage sags to 50% of the rated value must maintain the normal operation of the device for 0.2 seconds and the rest of the specifications are the same as that of the ITI.
  3. Voltage Interruptions: for short-term voltage interruptions, the ITI specification requires that the equipment must tolerate interruptions within 20ms and the equipment must still operate normally.
  4. Voltage swell: The causes of voltage swell include transient large load trips and system failures. Possible effects include illumination flickering, tripping of voltage-sensitive equipment, and equipment not functioning properly. The ITI specification states that 120% of the rated value must maintain normal operation of the device for 0.5 seconds.
  5. Low-Frequency Decaying Ring wave: Ringing caused by power factor correction (PFC) capacitors in AC power distribution systems. This capacitor-equipped device is called STATCOM or SVC in Taiwan Power Company, and its resonant frequency can range from 200Hz to 5kHz. To simulate this phenomenon, the AC power supply must have an output capability of 5kHz. The maximum ringing peak value is the percentage of the peak value of the rated voltage, 140% of the peak value; the frequency starts from 200Hz to 200% of the peak value; the frequency is 5kHz, and the peak value changes and the frequency changes are set in a linear increment. The ASR-3400HF launched by GW Instek in 2022 provides an output frequency of 5kHz, which can simulate the low frequency attenuation ring wave of ITI. 

     

    Figure 2 : Example of low frequency attenuated ring wave



  6. High-Frequency Decaying Ring Wave is the transient state of lightning strike. The AC power supply cannot directly correspond to this test. It is achieved by adding a "Lightning Surge Simulator". Please refer to ANSI/IEEE C62.41-1991 for the test conditions and methods. The countermeasure of the circuit is to use a surge absorber (TVSS5). The information in the ITI application document mentions that the equipment must have a transient surge withstand capability of at least 80 joules.

 


Figure 3 : Schematic diagram of power quality issues of no interruption in function region

 

 

 

Note 1： ITI is the abbreviation of Information Technology Industry Council

Note 2： CBEMA is the abbreviation of Computer and Business Equipment Manufacturers Association

Note 3 ： OVP Over Voltage Protection

Note 4 ： UVLO Under Voltage Lockout

Note 5 ： TVSS Transient Voltage Surge Suppressor

 

 

 

 

p.1  p.2  p.3

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