Isolated DC-DC performance in an ultra-small form factor

Equipment designs for advanced telecoms, networking, wireless infrastructure and storage continue to present new power-system challenges. It is not only that the systems at their cores – processors, ASICs and FPGAs – demand tightly-regulated supplies at a range of low voltages and high currents: the increased system-level performance metrics  – data processing capability or throughput –  are required to be achieved in less board area and smaller form factors. This increases the system value and differentiation versus their competition, and one key enabling factor is the ability to improve power-system densities, to free up incremental board space for system-level features.

Power supply densities and efficiencies already operate at levels that would have seemed impossible only a few years ago. Now a further step-function improvement has become feasible, thanks to the release by Picor, a subsidiary of Vicor Corporation, of the first of its Cool-Power® Isolated DC-DC Converters. 

The PI3101 is a 3.3V regulated output, 60W, ultra-high density isolated DC-DC converter, yielding an unprecedented power density of 400W/in³ and 105 W/in² while operating across a wide input voltage range of 36V-75V. Its high efficiency soft-switching power architecture combined with an innovative integrated Power-System-in-Package (PSiP) packaging concept, allows the power density of an isolated converter to be increased by up to 3 times the power density produced in conventional packages that are more than double the size.

Blend the advantages of traditional DPA and IBA schemes

The features of the PI3101 allow power system designers to revisit power architectures. Today, the most common approaches to supplying power to telecom and networking systems are the DPA (distributed power architecture) or the IBA (intermediate bus architecture). For other systems such as higher powered computing systems and servers, experienced designers are discovering the efficiency benefits of bussing 48V or higher voltages across the system backplane and converting directly from 48V to the POL voltage level desired.

Figure 1a and 1b show typical DPA and IBA architectures. The DPA will convert incoming AC to a DC voltage which is either 36V-75V or a narrower range possibly from a regulated 48V (+/-10%). Each output load voltage is supplied directly by independent isolated DC-DC converters. This method usually results in relatively high costs and large board-area requirements but it typically yields highest relative overall system efficiencies due to single stage conversions for the output rails employed.

The IBA (Figure 1b) evolved in an effort to reduce board area and cost versus the DPA scheme: it converts AC to the same DC ranges as the DPA. For the wide range requirements a regulated or semi-regulated, isolated bus converter (IBC) reduces the high voltage down to an intermediate bus, from which narrow-range non-isolated point-of-load converters (NiPOLs) supply the loads. For the narrow-range case, the isolated DC-DC converter that supplies the NiPOLs can be either semi-regulated or even unregulated completely. 

The IBA can use smaller NiPOLs and a single isolated power stage resulting in a relatively smaller and lower cost supply versus the DPA approach. The small NiPOL can be located close to the point-of-load for better transient response. The IBA's downsides are that the overall system efficiency can be reduced as compared to the DPA because each voltage rail undergoes a double conversion process and the distribution losses increase by the square of the intermediate bus current. The intermediate bus voltage may also not yield the optimum efficiency for each NiPOL.

The introduction of the Cool-Power PI3101 offers the opportunity to combine the advantages of both topologies, while also providing step-function improvements in power density and size versus both approaches. Providing the ability to bring power directly to the point of load, which has been virtually impossible with conventional solutions, some system power configurations can benefit from either reduced size or fewer conversion stages. The designer gains the flexibility to place isolated power at the point of load virtually anywhere it is needed and drawing on any available 48-V rail, regardless of the topology of the bus architecture.

Power-switching principles

The PI3101 is based around a novel and proprietary power stage topology that uses double-clamped zero voltage switching (DCZVS), a description that the step-by-step explanation below will clarify. The PI3101 operates in discontinuous conduction mode and at frequencies approaching 1MHz. Figure 2 shows the switching configuration that enables DCZVS operation. The PI3101 uses four primary MOSFETS, a synchronous MOSFET, a power transformer and resonant clamp capacitor that together form the power delivery subsystem. Q1 and Q4 are power switches and Q2 and Q5 are clamp switches. Q3 is the synchronous MOSFET. The power cycle has six distinct stages of operation, as shown in Figure 3.

1. Transition to T1 from clamp phase: Q2 and Q4 are both on, Q5 and Q1 are both off. The clamp capacitor connected to the drain of Q5 is charged up to the reflected voltage which is equal to Vout * (Np/Ns). If the transformer core is properly reset, the minimum-period timer initiates Q2 turn-off. If additional reset time is needed, the minimum period is extended until core reset is complete. 

The clamp current flows as long as Q2 and Q4 are both on, and transitions to magnetizing current as Q2 is turned off and volt-seconds are developed across the primary winding. This magnetizing current must continue to flow and while it does, the VS1 node rises towards Vin as the D-S capacitance of Q2 charges and Q1 discharges. As VS1 rises, the proprietary controller begins monitoring of primary volt-seconds.

2. T1 Power stroke phase: Q1 turns on at virtually zero voltage and the primary current ramps up in the transformer, storing the required energy determined by the internal error amplifier, as a function of load current and line voltage. At this point, Q1 and Q4 are both on. The controller monitors primary volt-seconds.

3. Transition to T3 phase: Q1 and Q4 are turned off very rapidly. The proprietary combination of driver and MOSFETs allows this transition to occur with virtually no losses. Both the VS1 falling edge transition and VS2 rising edge transition, are very fast, and prepare the circuit for power transfer from primary to secondary. All switches are off at this time. (Figure 3)

4. T3 Power delivery phase: Q2 and Q5 turn on, followed by the synchronous MOSFET Q3. Q3 turns on (and off) at the optimum time for lossless switching as dictated by the intelligent gate driver, and by the resonant circuit formed by the transformer inductance and the clamp capacitor. The secondary current ramps up as the stored energy in the transformer is released to the load and output capacitors.

5. Clamp-phase transition: At the end of T3 when the energy has been delivered to the load, Q3 is switched off by its driver. The main controller monitors the primary volt-seconds and determines when transformer reset is complete, then turns Q5 off. VS2 falls and allows for the zero-voltage turn-on of Q4.

6. Clamp phase: Q2 and Q4 are turned on again: The output voltage is monitored through a (patented) sampled-feedback interface and processed by the PI3101's control circuitry. The next cycle is started by the expiry of the minimum period timer. The energy remaining from the clamp phase is used to zero-voltage-switch the VS1 node.

The combination of the DCZVS buck-boost topology, very low-leakage-inductance planar magnetics, high-performance MOSFETs and advanced controller IC, in high-density surface-mount PSiP (Power-System-in-Package) format, results in a 22 x 16.5 x 6.7mm device that looks more like an IC than a power supply. 

Sampled feedback control eliminates optical feedback and simplifies compensation, and the low-profile outline offers the simple implementation of a number of cooling options. Other features include +/-10% output voltage trimming with an external resistor; soft start with an external capacitor; 5V reference output; temperature monitor; and an array of protection features.

The PI3101 achieves up to 3 times the power density of conventional 1/16th bricks currently available. It can match a typical 1/16th brick in terms of average load capabilities, transient response, output accuracy and efficiency, in less than half the footprint.

Re-architecting frees PCB space

Take as an example the same system shown in Figure 1a; before the advent of the PI3101, the smallest isolated approach would be to use four 1/16th bricks. Keeping the same architecture but simply replacing the four 1/16th bricks by (forthcoming) variants in the PI310x Cool-Power family, the total system size could be reduced in half. Meeting the demands – around 130W – of the system in Figure 1a with a fixed-bus IBA configuration, assuming a wide range 36-75V input bus, indicates an isolated converter of 1/8th-brick size plus four NiPOLs. 

Re-configuring this architecture with isolated PI310x’s eliminates the 1/8th-brick and the equivalent of two more NiPOL units. High current, high density NiPOLs could still be employed for the 1.8V and 1.2V rails, stepping down from the 3.3V output of a PI3101, for an overall board area saving of up to 75%.

Embedded solutions

Purchasing a suitable controller IC and constructing a discrete isolated DC-DC power supply that exactly meets the requirements of a target system is always an option, but usually requires a lot of design time, resource and expertise. Typical state-of-the-art switching controllers on the market today have accompanying reference designs, but they can be very high component count and considering the recommended  transformer alone it is  larger than the entire PI3101 solution – before the entire parts-count of some 45 components is placed and routed. 

The PI3101 (Figure 4) integrated solution eliminates the entire development process that the discrete embedded option would entail including the corresponding safety certification process, reliability and manufacturing optimisation. Specific application areas for which the PI3101 is an immediate fit include telecom and networking and communications equipment operating from a 48V input bus, including the new high power standard of PoE – a single Cool-Power solution handles the maximum allowed power supported by the standard.

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