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How To Use Pre-Switch System Optimizations

May 31, 2018

Pre-Switch Technical overview:

 

Pre-Switch technology brings four fundamental benefits to the power conversion industry and delivers a step function in improvement in power converter cost, size, weight, efficiency, and reliability. This article describes the options and trade-offs in Pre-Switch implementations. Benefits 2-4 are designed to support higher switching frequencies and enable new levels of safety by using our AI monitoring capabilities.

 

 

Four Fundamental Benefits:

  1. Switching loss reduction of 70% to 95% on any switch type or topology

  2. Drastic EMI reduction

  3. Select the dV/dt required for your application 

  4. Pre-Switch Blink: a new cycle-by-cycle safety monitoring system built into the Pre-Flex chip

Basic Operation:


To understand Pre-Switch technology it is important to understand the basic operation of our auxiliary forced-resonant architecture. (See image at right; Pre-Switch architecture with added components in Green) 

 

 

 

Soft-Switching:
 
Pre-Switch technology enables soft-switching by forcing an auxiliary resonant circuit to briefly collapse the voltage (or stop current flow) across the target power transistor. The result is next to zero overlap between the current and voltage switching wave forms. (see "Pre-Switched" image at right: a Zero Voltage (ZVS) soft-switching waveform showing no overlap in current and voltage). This is achieved by “Pre-Switching” a separate, small, low-cost transistor (switch) with precise timing. Our system uses a low-cost IGBT to soft-switch high-speed SiC systems. The timing—how and when to switch—is learned and adjusted in-system by the Pre-Flex embedded AI chip. Pre-Flex learns on a cycle-by-cycle basis and effectively guarantees soft-switching, no matter what the topology or changes in input voltage, output conditions, temperature, or manufacturing tolerances. The result is that the target transistor is turned on and off with a 70-95% reduction in switching losses. The variation in switching loss reduction is dependent on transistor type, package inductance and how hard the transistor was being driven prior to being Pre-Switched.


The Pre-Flex architecture reliably enables a shorter blanking period for each transition between the high side and low side transistors in a half bridge vs. hard switching. This has the effect of producing a higher modulation index at any given switching frequency.  These higher modulation indexes allow designers more useful range of the given DC voltage.  Unlike other faster switching transistors, the Pre-Flex architecture enables the selection of any dV/dt and the use of lower cost transistors such as IGBT to achieve the same switching frequencies as SiC and GaN devices.


Transistors with reduced switching losses can be switched faster at the same loss levels. Pre-Switch refers to this figure of merit (FOM) as an X-Factor. The X-Factor is how much faster a system can switch because of the reduction in switching losses enabled by Pre-Switch soft-switching. Figure 3 shows Pre-Flex’s ability to eliminate 80% of an IGBT’s switching losses while switching five times faster. This is a demonstration of an X-Factor of 5. Pre-Flex has shown to eliminate 95% of a Silicon Carbide MOSFET’s switching losses and switch that device at 1 MHz with industry-leading efficiencies (that’s an X-factor of 20). 

 

 

 

 

 

 

 

 

The fundamental value propositions of Pre-Flex technology

Pre-Flex technology reduces EMI in the system because the target transistor has little to no voltage across it (Vce, Vds) when it soft-switches. The result is a large reduction in EMI emissions compared to hard-switched topologies. Further, our auxiliary circuit places a large capacitor across the target transistor, which slows the rising and falling edge speeds during the turn-on and turn-off transition. This is done to assist the forced resonant circuit and to reduce the dV/dt generated by hard-switched circuits. Both of these features have cost- and functional benefits for system designers.


Pre-Switch Blink™, is a series of fast cycle-by-cycle safety features that intelligently shut down a power converter in case a fault is detected. Blink is integrated into the Pre-Flex platform, and it provides cycle-by-cycle over-current protection for each switch independently. Blink uses a communications port built into the Pre-Flex chip to provide error codes and other related communications to an outside host.


Options for System Optimization 

 

Designers can allocate Pre-Flex’s switching loss-savings in two directions for new systems:

  1. Keep the same switching frequency and exploit the reduced losses. Referred to as the EFFICIENCY condition. 

  2. Keep the losses the same and exploit the increased switching frequencies. Referred to as the COST condition. 

Both options allow further sub-options, and each can be used for a jump in system performance never before offered by new transistor technologies.


EFFICIENCY: System optimization with the same switching frequency
 

  1. Designers can use Pre-Flex switching loss reductions to increase system efficiency. As an example, In systems with approximately equal switching losses and conduction losses, system losses can be reduced by 40%-47%. The increased efficiencies lower the heat sink size and costs while saving wasted power. The heat sink savings, case savings and packaging savings lower the $/W at the system level well in excess of the cost to add the Pre-Switch architecture.

    Example: A power converter is 95% efficient. Half of the total losses are switching losses. Reducing those losses by 80% yields and a new efficiency of 97%.
     

  2. Designers can keep the heat sink size the same and efficiency the same, but significantly increase the amount of power the power converter can convert. This also reduces system costs because the same internal components of the previous power converter is now elevated to a higher power level –lowering the $/W at the system level.

    Example: a 10-kW power converter that is 95% efficient. The same system with slight modifications for larger wires and current carrying non-transistor components in the filter can now convert in the vicinity of 13 kW.
     

  3. The third option can be any combination of the above. We encourage designers to think of Pre-Flex technology as steroids for their transistors, allowing them to run faster and carry a higher load than ever before.
     

COST: System optimizations with higher switching frequencies
 

  1. Keeping total switching losses the same exploits Pre-Flex’s ability to switch any transistors significantly faster. Pre-Flex technology also reduces the blanking time between transitions and can be designed to reduce the dV/dt to any level required for the application. No longer do designers have to sacrifice motor reliability to switch faster for improved motor efficiencies and cost reductions in the motor drive. Switching faster shrinks the passive components size, cost and weight. Generally, a 5X increase in switching frequencies means that passives shrink to 1/3 of the original size and weight AND drop in cost by 50%. This option has proven to significantly lower system cost, size and weight and therefore results in the largest decrease in $/W. Higher switching frequencies have many other benefits.

 

Example: Higher switching frequencies reduce output ripple in motor drives which is the dominant loss in motors at low torque.
 

Example: Higher switching frequencies can be used to enable a low cost output filter that was previously not possible. Now, for the first time motor drives can be built around a low-cost output filter –further increasing motor efficiency.
 

Example: Higher switching frequencies push switching frequencies above human audible detection. Have you ever noticed the whine of the power inverter in an EV car when you accelerate rapidly?  That problem goes away with Pre-Switch technology.
 

Pre-Switch technology has started a new race between power converter companies willing to break free from hard-switching constraints. We invite you to join us and to advance your leadership in power converter cost, efficiency, weight, power density, EV range, reliability and motor size.

 

Please contact sales@pre-switch.com to learn how reducing switching losses can benefit you.
 

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