Aspects of AESA Power Supply ManagementMarch 26, 2019 by Oleg Negreba
When developing a conceptual design of a power supply units for different pulse loads, designers can use two different approaches – either to use a high
When developing a conceptual design of a power supply units for different pulse loads, designers can use two different approaches – either to use a high power PSU which provides full impulse power or to average the pulse power by a big capacitor bank and use a PSU of less power.
Part 2: Is it Necessary to Average the Pulse Load of the PSU?
Here we will consider design examples of power supply systems for any pulse loads with pulse power averaging and without it; we will also analyze approaches to selecting PSUs, show features, advantages and disadvantages of both configurations.
Figure 1 shows the structure of these systems and Figure 2 shows current forms supplied by PSU and consumed load, and also illustrates its voltage.
Vicor Corporation, a manufacturer of modular PSUs, has shown  that in many cases a power supply system is more compact, cheaper, and more reliable with averaging of power consumed by pulse load. Such an approach is the most effective for applications where the load allows essential voltage instability caused by discharge and charge of bulk capacitor connected to it in parallel. Despite that, several sources [3, 4] show it is necessary to use capacitive or inductance-capacitive energy storage at each transmit-receive module (TRM) when building any power supply system, in some cases it is possible to decrease their rating and sometimes completely exclude such capacitors from the structure, herewith improving weight-and-size and reliability parameters of the power supply system .
Let us consider one typical example of an AESA TRM power supply system. TRM power supply voltage is 28VDC, consumed pulsed power is 450W, the maximum pulse duration is 5ms, off-duty factor – 5 (that corresponds to load factor of 20%). The following table gathers initial data of such requirement definition.
In the first case for the power supply of AESA TRM we will use a PSU with a power of 500W, which will provide full 450W power consumed by the periodic pulse load. In the second case, due to the fact that average load power is only 90W, we will use a power supply unit of 150W together with a large bank of capacitors for averaging the current consumed by the load. Because the microwave radiation indexes of TRM transceiver power strictly depend on stability of voltage the typical demand on this voltage instability is ±1...4 %.
To provide the specified quality of power supply load voltage in configuration without averaging of pulse power it is enough to have one small capacitor with capacitance about 220uF, operating only during step transients, and for system with power averaging it is necessary to have the whole bank of capacitors with total capacitance about 70000uF, providing load with power during its complete operating pulse. There are several problems, which designers of electric power supply systems could face using a capacitor bank of such size . In particular, many serial modular PSUs when turned-on at such high capacitance causes the converter to trip output overcurrent protection, besides, capacitor bank of large power can destabilize the voltage feedback circuit in the PSU.
For the circuitry without pulse power averaging the value of required power capacitor at PSU output directly depends on its voltage-feedback circuit performance that is mainly limited by PSU switching frequency – high switching frequency allows receiving a quick response to load stepped variation.
Documentation of unified modular voltage converters often contains transient waveforms to illustrate output voltage reaction to pulse load; these waveforms help to compare the response time of their feedback to the dynamic load. When analyzing the value of transient output voltage deviation on steps of the current of such waveforms, it is necessary to pay attention to the range of load current change in percent from maximum PSU power to minimum value of dynamic load during tests and also pulse current rate of change.
Most of PSU manufacturers carry out dynamic load tests of their products in rather gentle conditions to demonstrate acceptable output voltage stability, for example, they set range of PSU load variation only as 25% or 50%, perform load drop not to no-load, but set rather slow current change fronts with duration in dozens and thousands of microseconds. None of the listed approaches to tests is not acceptable in applications with TRM power supply without pulsed power averaging, i.e., output current of PSU changes in micro and even nanoseconds, power supplied by load during pauses is close to zero, but it is 70% to 100% of maxi-mum PSU power during operating pulse. For unbiased evaluation of processing quality by pulse load converter, you either have to request the manufacturer to send you transient waveforms of converter output voltage received under required mode of operation, or to perform such measures by own means.
Figure 3 shows a comparison of two different mass-produced isolated voltage converters with power 500W and output voltage 28VDC with a capacitor of 220uF connected to their output, and load step from zero to 100% and inversely with current change fronts of 200nsec. Under the same conditions, the value of transient output voltage deviation with a load power surge of fast feedback converter is approximately fifteen times less than of similar non-dedicated converter. High-speed performance of voltage feedback circuit and capacitance value of own output capacitors of the second converter were enough to provide 3% stability of output voltage without any serious external filters under given conditions.
Let us compare the PSU set-ups for configuration with averaging and without averaging of pulse power. Because more powerful converters usually have higher efficiency, then the considered example shows that 500W converter efficiency with output power of 450W is 92% and causes heat generation of 7.8W, while the efficiency of 150W converter chosen for the system with averaged pulse power at average load power 90W is equal to 87.5 %, which causes heat generation of 12.5W. As a result, the converter surface is enough for the converter of higher power to operate without overheating, but the converter of 150W needs an additional heat sink and forced air cooling. Even when a PSU is mounted used with a liquid cooling system along with the TRM, excessive heat generation induces undesirable performance reduction.
Thus, we can see that the attempt to save on PSU power causes essential difficulties for the power supply system and makes the use of such configuration significantly more complicated (Figure 4).
Figure 5 shows the experimentally recorded waveforms, which il-lustrate on-load voltage form and consumed current form for the first and the second variants. In both examples form of current supplied by load is equally pulsed and rectangular which is seen by lower waveforms, but in configuration with averaged load power there is vivid sawlike component induced by discharge and charge of storage capacitor battery, but in the second structure, without output volt-age averaging, the form of voltage is almost a straight line with short transient processes at points of load ON and OFF.
The following waveforms show forms of current given by converters (Figure 6). It is seen that the first converter really operates with aver-age power near 90W, but the second converter gives power pulses to the load with amplitude 450W.
In is important to notice that output current form of converter, shown on the waveform (Figure 6b) points to one of the disadvantages of the power supply system for pulse load without averaging. A PSU of given configuration provides proper load power supply during the complete operating pulse, the current consumed from input mains also has an expressed pulse character that is not acceptable in every applica-tion. Input load in the example with pulse current averaging is several times less.
If the input mains is able to provide complete load pulse power with account for power supply unit efficiency, then usually there are no problems with using configuration without pulse power averag-ing, but in case when its power is limited and doesn’t allow to supply load with proper pulse power, it is necessary to take special measures to convert consumed current type from pulsed to averaged with some permissible ripple by means of input active or passive current filters.
During load pulse, such a filter must limit the current consumed from input mains but at the same time it must supply the required pulsed power to PSU, while in pauses is should use the mains to replenish the difference between yielded and consumed power. It is obvious that such devices, being a current filter, should perform an energy storage function.
Use of passive inductive or capacitive energy stor-age is almost always results in unacceptable weight-and-size and cost parameters, therefore, the best solution is to use high frequency voltage converters without galvanic isolation with filtering output capacitor as a current filter. Moreover, it is advisable to use step-up converters instead of step-down ones, because usually it is energeti-cally favorable to accumulate energy with higher voltage as accumu-lative energy in capacitor is proportional to voltage squared (E = CU2 / 2). In many cases, if the initial system is supplied from AC mains, such filter function will be taken by a power factor corrector.
So, what do we have in the end? To construct a pulse load power supply system without averaged pulsed power we need one converter with a power of 500W with a small output capacitor. Nevertheless, we have to be sure that input mains can withstand pulsed consumption with an amplitude of about 500W. To construct system with averaged pulsed power we need additional accessories apart from the converter for smoothing the output pulsed current, as well as the effective heat removing system that causes reduced reliability of such configuration, i.e., electrolytic capacitors dry out in time, and the electromechanical component – the cooling fan – having limited lifetime is exposed to dust and sand and generates noise and vibration.
When using liquid cooling system, excessive heat losses decrease its performance. Furthermore, if during system operation or upgrade the pulse duty factor of the load current will be increased from 20%, to, for example, 40%, that will not be reflected in the operation of the 500W converter, while the 150W converter will go into the emergency overload mode and may become faulty.
If we compare the price of the two considered configurations of power supply systems, that will leave almost no chances for the variant with power averaging. As a case study of advantages of approach to pulsed load power supply without pulsed power averaging, Figure 7 shows a visual configuration of a 12-channel power supply system , supplying energy to eight TRMs with voltage +28VDC, and to service units with voltage +5 VDC, +12VDC, -6VDC and -50VDC.
Capacitors installed at +28VDC outputs support required voltage only during transient load steps and do not influence pulse current range. In addition, this device was accomplished with the concept of TRM electric power supply decentral-ization, which helped to obtain the high reliability, low height (11mm) of the system, as well as other advantages typical for decentralized electric power supply systems. Pulsed load impact to primary AC mains supply is blocked at the most useful place, i.e., from the side of high voltage of capacitor bank connected to the output of the primary converter forming a bus bar +300V from AC input voltage.
Thus, the presented analysis shows that there are certain require-ments to AESA power supply management to satisfy which the systems without pulsed power averaging will be essentially more advantageous. However, there is a range of applications where aver-aged power value would be more favorable. First, in structures with a significant off-duty factor, i.e., for pulse duty factor less than 10%, or with the pulse duration lasting microseconds or fractions of microsec-onds, rather than than milliseconds, as in the considered example, i.e., the time compared to voltage feedback response time. One more possible application for systems with averaged power are the systems when the converter load can withstand a wide range of input voltage, e.g., POL-converters.
In each specific case, a designer of AESA power supply system should pay attention to all available factors, consider all requirements of the work statement for the system design and decide about its con-figuration after studying the entire range of advantages and disadvan-tages of all possible implementation variants.
The author expresses sincere gratitude for help in material prepara-tion for the article to the Chief Designer of “ELLIARS” LLC M.V. Sye-din, Deputy General Director of АО “NPP “Radiy” by scientific work, to Chief Designer M.G. Vitkov, project engineer, the head of section АО “Microwave systems» A.D. Matveev, leading engineer of “Almaz-Antey Telecommunications” LLC A. V. Gurin, the head of department АО “VNIIRT” Volodin I.N. and also to employees of “AEDON” LLC N.V. Chetverikov, A.N. Protsenko, D.S. Ermakov