High-Voltage Energy Storage: the Key to Efficient Holdup Jean Picard
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High-Voltage Energy Storage: The Key to Efficient Holdup Jean Picard ABSTR A CT This topic provides a tutorial on how to design a high-voltage-energy storage (HVES) system to minimize the storage capacitor bank size. The first part of the topic demonstrates the basics of energy and the benefits/limitations of HVES; it also provides volumetric analysis graphs illustrating volume reduction and energy density. The second part provides an overview of the critical aspects of an HVES design. It compares the possible topologies and control techniques, identifies the pitfalls and design challenges of the recharge and holdup modes, and discusses the impact of design decisions on power losses. Design guidelines applicable to a preselected power topology and control strategy are also provided, including a design example for a 48-V application. I. FUND A MENTA LS O F HI G H -VOLTA GE holdup event such as a momentary input-power ENERGY STOR A GE interruption. Fig. 2 shows typical Vbus levels with A. Basic Principles and without an energy-storage system. For example, in telecommunications applications, the Many high-reliability systems have a require- ® ® ment for modules to ride through short input- PICMG AdvancedTCA specification requires power interruptions. Some systems require a continuous operation in the presence of a 5-ms, graceful shutdown mechanism while others need a 0-V input-voltage transient (the total duration is local bank of energy to supply power during 9.2 ms when rise and fall times are included). The occasional and brief high-load-current demand. 5 ms is the time in which a fuse can blow when There are also other applications that require there is a short circuit on a module or a short- short-term backup power when the main power circuited module is plugged into the system. Many fails—for example, a security system that needs to other similar examples can be enumerated, both record information for a limited time following a for DC and AC bus voltages. power interruption. 5 Topic Vbus t Input-power interruptions can come from a holdup short-circuited module or from the switchover Vholdup from a primary to a redundant bus when a power- Without Energy bus failure occurs. Fig. 1 shows a basic energy- 0V Storage storage system that could maintain V during a bus Fig. 2. Bus voltage during holdup event. Bus #1 Hotswap Regulated Input DC (Optional) DC/DC Rectifiers Vbus Voltage Voltage + Converter to Load EMI Filter Bus #n Energy-Storage System Capacitor Bank Fig. 1. Bulk-capacitors solution for energy storage. 5-1 One way to ride through the fault is to use Rectifiers + Regulated storage capacitors. The Input DC/DC Hot Swap Voltage energy available is Voltage Converter (Optional) to Load defined as + EMI Filter HVES System 1 2 2 EC=−×()VV1 2 , (1) 2 High Step-Down Voltage where E is the energy Regulation at in joules (J), C is the Vholdup capacitance in farads (F), V1 is the starting Capacitor capacitor voltage before HVES Bank Recharge at V discharge, and V2 is the 1 final capacitor voltage after discharge. The greater the voltage decrease, the smaller is Fig. 3. HVES Solution. the capacitance required to hold up the circuit. 2.5 In a bulk-capacitors solution (Fig. 1), energy is stored in capacitors on the power ) Panasonic EEFK 2 2 bus. This requires a large capacitance value NCC KZE kJ/m because the allowed voltage decrease is ( usually a small percentage of the bus 1.5 voltage. An alternative solution, high-voltage- Density 1 energy storage (HVES) stores the energy on a capacitor at a higher voltage and then Energy 0.5 transfers that energy to the power bus during Topic 5 Topic the dropout (see Fig. 3). This allows a smaller 0 capacitor to be used because a large 100 80 63 50 35 25 16 10 percentage of the energy stored is used for Cap Voltage Rating (V) holdup. HVES is a particularly good choice Fig. 4. PCB energy density with V2 = 39 V and HVES when the bus voltage has a wide range of solution with V1 = 88% of capacitor rating. variation (for example, a 48-V bus with 72 V maximum), already requiring high-voltage bulk If, for example, 2 J are required over a 10-ms capacitors; or the minimum normal bus voltage is time period, V2 is 39 V, and V1 is only 44 volts, not much higher than the minimum needed by the then 9639 µF is required. If V1 is increased to loads to stay in operation (for example, 44 V 88 V and the power-conversion efficiency is 91%, versus 39 V). the required capacitance decreases to less than 706 µF, which means a reduction factor of close to B. Volumetric Design Examples 14. In both cases, some derating must be applied In Fig. 3 and Equation (1), the capacitor bank to the required capacitance. This includes the has initially been recharged and its voltage is V1. initial tolerance, the temperature variation, and the V2 represents the capacitor bank’s minimum lifetime variation. voltage required to maintain Vholdup, which is the Figs. 4 and 5 illustrate how increasing the regulated bus voltage during a holdup event. storage voltage reduces the PCB area, assuming 5-2 30.5 Square Inches 24.8 Square Inches 4.86 Square Inches HVES TopView 1.8 Inches 6.1 Inches 5.5 Inches 2.7 Inches HVES Bottom View 5 Inches 4.5 Inches a. Bulk-capacitors solution b. Bulk-capacitors solution c. HVES solution with with V=1244 V, V=39 V, a with V=1244 V, V=39 V, a V=1288 V=39 V, a 200-W 200-W load, and 330-µF/100-V 200-W load, and smaller load, and 330-µF/100-V capacitors. 330-µF/80-V capacitors. capacitors. Fig. 5. PCB area with HVES solution versus bulk-capacitors solution. that the capacitor’s height is no more than 21 mm Fig. 6 illustrates the energy density achievable and the capacitor’s final voltage (V2) is 39 V. with a bulk-capacitors solution that uses the same Capacitor types considered are the Panasonic final voltage and capacitor types. The starting surface-mount EEFK and the Nippon Chemi-Con capacitor voltage has been set to 44 V, and the (NCC) leaded through-hole KZE. The storage capacitor voltage rating varies depending on the voltage has been arbitrarily set to 88% of the maximum expected bus voltage during system maximum capacitor voltage rating, the capacitance operation. derating factor has been set to 74%, and the conversion efficiency has been set to 91%. 0.35 5 Topic 0.3 Panasonic EEFK ) 2 NCC KZE 0.25 kJ/m ( 0.2 Density 0.15 0.1 Energy 0.05 0 100 80 63 Cap Voltage Rating (V) Fig. 6. PCB energy density with V2 = 39 V and bulk- capacitors solution. 5-3 2.5 0.08 Panasonic EEFK 0.07 Panasonic EEFK ) ) 2 2 NCC KZE 2 0.06 NCC KZE kJ/m kJ/m ( 1.5 ( 0.05 0.04 Density Density 1 0.03 0.02 Energy 0.5 Energy 0.01 0 0 100 80 63 50 35 25 50 35 25 16 10 Cap Voltage Rating (V) Cap Voltage Rating (V) Fig. 7. PCB energy density for 12-V system and Fig. 8. PCB energy density with V1 = 11 V, HVES solution with V1 = 90% of capacitor rating V2 = 10 V, and bulk-capacitors solution. and 70% conversion efficiency. Analysis results for a 12-V system having a impact on the system. It is also highly preferable conversion efficiency of 70% are shown in Figs. 7 to make use of a single inductor for all modes of and 8. It is clear that storing energy at 50 V is operation for a compact system solution. EMI enough to provide a high size-reduction ratio. requirements, thermal aspects, and total cost must also be considered. II. CR I T I C A L ASPECTS O F HVES IMPLEMENTAT I ON B. Optimum Power-Bridge Configuration and A. General System Requirements Control Strategy for Bidirectional Converter Basically, an HVES system must be able to: A simplified system diagram is shown in Fig. 9, which presents the HVES like a virtual • use the bus voltage to charge and maintain the capacitor. While the key to minimizing the size of storage capacitors to a nominal voltage, the HVES unit is high-voltage storage, conversion • use the energy available in the storage capacitors efficiency also has some impact. to quickly maintain and regulate the internal input bus voltage during a short input-power Topic 5 Topic interruption, • automatically dis- Rectifiers + DC/DC Regulated charge the storage Input Hot Swap Converter Voltage Voltage (Optional) + ca pacitors on unit to Load + HF Input re moval for safety, and EMI Filter Filter • minimize the size of Simple 2-Wire Connection HVES System the storage capacitor bank. High Holdup Voltage Regulation Secondary needs include inrush-current control during initial Storage Capacitor recharge, short-circuit Recharge Bank pro tection during Capacitor Bleed- recharge and very low Down power consumption when the capacitors Control become com pletely charged for minimum Fig. 9. Simpified HVES functional diagram. 5-4 The storage capacitor Rectifiers DC/DC bank is connected only to + Converter Input Hot Swap + the HVES power-bridge Voltage (Optional) HF Input + circuit. It is also normally Filter physically located very close EMI Filter to this circuit, with Simple 2-Wire Connection enclosure(s) or other types HVES System of barriers that bar user High access to satisfy safety Holdup Q Voltage Regulation D concerns.