Hard-Switching, Soft-Switching, Pre-Switching

IGBT transistor schematic


Transistors were invented in the 1940’s and today come in a variety of devices used in power converter such as BJTs, thyristors, MOSFETs and IGBTs. At a logic level, transistors are simple, three-pin switches in which the application of voltage or current into the third pin enables current to flow between the other two pins (see top image). When the transistor is turned on or off, the transition time needed to achieve the next state is very short, but it is not instantaneous. The time a transistor needs to fully transition between on and off states produces wasted energy, known as a switching loss (see image: Traditional Hard-Switched). Switching losses occur at the intersection of the voltage and current wave forms. Switching losses are responsible for a large percentage of power converter losses. The amount of switching losses each transistor has varies by transistor type, manufacturer and operating voltages. In general, transistors that can handle higher voltages have larger switching losses.

Hard-Switching a transistor causes switching losses when the current and voltage wave forms overlap.

Hard-Switching is simply forcing the transistor to turn on and off (commutate) by adding the current or voltage to the third pin to enable the changed states. Hard switching is known to be hard on transistors and shortens’ their useful life. Hard switched transistors in large wind turbines and electric trains are designed to be pulled out of the system and be replaced as a form of maintenance.

Hard switching is easy to understand and therefore is used almost universally in power converters outside of the low power DC to DC space. Hard switching is thought to be low-cost because only a limited number of other components are required to drive the transistors between states. In reality, hard-switching is expensive at the system level and inefficient.

Hard Switching half bridge

Hard-Switching has numerous well-known drawbacks, the largest of which is the introduction of switching losses. Power converters using Hard-Switching (again, virtually all of them), need to balance the desire for higher switching frequencies with the need for acceptable system losses to meet the desired system efficiencies. In practice this means that systems requiring high efficiencies are designed to switch slowly to gain efficiency. The efficiency gains come from reducing the cumulative amount of switching cycles (and hence cumulative switching losses) of each transistor in the conversion process. The drawback of this approach is that designers must increase the size of other components in the system required to hold the power for a longer time period between the transistor’s longer switching cycle.

There is another drawback to reducing a transistor’s switching frequency in order to gain efficiency. By slower switching frequencies produce high harmonic distortion and output ripple. Distortions and ripple generally need to be filtered to make the power usable. Power designers typically solve this problem by adding larger output filters -which again adds cost, size and weight.

Hard-Switching artificially limits the maximum switching frequency at which transistors can be commutated (switched on and off). Each transistor has a limit of the amount of heat it can dissipate. This maximum thermal dissipation must be balanced between conduction losses and switching losses for the desired outcome. Increasing the switching frequencies to reduce the size of a system means that the transistor has to carry less working current to accommodate the higher switching losses. This can be solved by adding a larger transistor with less conduction losses –but this also adds costs to the system. In short, with hard-switching, the switching losses generated by switching faster means less conduction losses can be accommodated. Without the losses hard switching creates, transistors would be free to switch much faster or handle more current for the same thermal limit.

Finely, Hard-Switching creates electromagnetic interference (EMI), which further increases system costs because of the need for additional components and shielding to meet various international standards such as the FCC class ***X99, EUXXX and JP.

The key take-away is that Hard-Switching transistors yields inefficient systems with increased cost, size and weight.


Zero Voltage Soft Switching with no overlap in current and voltage wave form

The concept and term “Soft-Switching” was introduced in the 1980’s by Deepakraj Divan who is now at Georgia Tech as the Director of the Georgia Tech Center for Distributed Energy.

His idea was to use an external circuit to prevent overlap of the voltage and current wave forms during transistor commutation. Today, there are two types of soft switching: a) self-resonant and b) forced resonant. In self-resonant Soft-Switching, a self-oscillating circuit is used to precisely time transistor commutation, resulting in the offset of the current and voltage wave forms (Figure 4). The benefits of a self-resonant Soft-Switching topology is the elimination of switching losses, an increase in efficiency and the reduction of EMI. The main drawback of self-resonant Soft-Switching is that the architecture only works in non-isolated power converters when input voltage and output loads stay within a narrow range. The result is that self-resonant Soft-Switching is used in a small portion of the power converter market for DC/DC converters.

1998 $45M US Navy Research project to attempt forced resonant Soft-Switching 100kW, 60kHz 3-ph inverter using a high-end DSP controller, custom power modules, and liquid cooling. The project was considered too expensive, complicated and provided limited benefits

Forced-resonant soft switching topologies use numerous inputs (input voltage, load, transistor voltages, currents etc.) to calculate the timing required to force a resonance to offset the current and voltage wave forms –thereby eliminating switching losses. Forced-resonant soft switching has the same benefits as self-resonant soft switching with the of elimination of switching losses, increasing efficiency and the reduction of EMI. Forced-resonance systems once held the promise of being able to be used on all power converter topologies, not just DC/DC where self-resonant soft switching is used. But forced-resonant Soft Switching has minimal market share and has fallen out of favor for future development because it is computationally challenging, cumbersome and had limited adaptability across varying input conditions and load ranges. The result is that forced resonant soft switching never lived up to the promise.


Pre-Switch, Inc. solved the issues of computational limitations, cost and complexity that previously prevented forced-resonant Soft Switching from success. Pre-Switch technology is based on an embedded Artificial Intelligence (AI) integrated circuit (called Pre-Flex) which precisely controls and adjusts the timing of a very small and low-cost resonant circuit to ensure that there is minimal overlap of current and voltage waveforms of the switching devices. The Pre-Flex IC learns and adapts in-circuit on a cycle by cycle basis to guarantee optimal soft switching. Pre-Flex locks each transistor into a forced resonant soft switching despite changes in input voltages, output loads, system temperatures and manufacturing tolerances. Pre-Flex driven forced-resonant technology have been documented to eliminate 70-95% of total switching losses (hyper link to hard data),The technology also significantly reduces EMI because there is virtually no power radiated during transistor commutation. Further, the technology can be used to enable virtually any desired dV/dt per switching cycle -which is a large enabler for the newer faster switching wide band gap devices.

simple, low-cost additions to enable Pre-Switching

Pre-Switches technology uses an auxiliary resonant circuit wrapped around the switching transistors enabling soft switching in virtually any power converter topology (see above). A minimum number of passive components,an active switch and the Pre-Flex IC is all that is required accurately soft switch virtually any power level. The power required by Pre-Switch’s auxiliary resonant circuit including the Pre-Flex IC is between 1-4% of the total switching losses saved. The technology has been used to switch 600 V IGBT at more than 100kHz and 900V Silicon Carbide transistors at 1 MHz with unprecedented system efficiencies.

The cost to add Pre-Switch’s auxiliary resonant circuit and the Pre-Flex IC are insignificant when compared to system-level savings. System level savings are dominated by the virtual elimination of switching loss but are further enhanced by reduced electromagnetic interference and a designable dV/dt.

Designers can allocate Pre-Flex’s switching loss savings in two directions for new system optimizations: 1) Keep the same switching frequency and exploit the reduced losses. 2) Keep the losses the same and exploit the increased switching frequencies. Both of these options have further system level options and benefits (see below).

Pre-Switch describes the ability to switch a transistor faster an X-Factor. X-Factor is the amount faster a system can switch because of the reduction in switching losses enabled by Pre-Switch soft switching. As shown above, we use Pre-Flex’s ability to eliminate 80% of an IGBT’s switching losses to switch 5 times faster. This is a demonstration of an X-Factor of five. Pre-Flex has been used to eliminate 95% of a Silicon Carbide MOSFET switching losses (X-factor 20).

In preparation to change the power converter market, Pre-Switch developed Pre-Switch Blink and integrated it into the Pre-Flex chip. Blink is a series of fast cycle by cycle safety features that intelligently shuts down a power converter should there be fault detected. Blink further uses the integrated a communications port built into the Pre-Flex chip to provide error codes and other related communications to an outside host on a cycle by cycle basis.

Pre-Switch technology can be used to upgraded hard switched systems already in the field. Pre-Flex has been integrated into a standard gate driver board for a 1200V 225A EconoDUAL in a half bridge configuration. Future gate driver boards are available to be custom developed upon customer request.

Pre-Switch technology is poised to set off the next big technology race between power converter companies, and we are excited to be at the forefront of innovation in all the industries that are dependent on more efficient power converters. Come join our revolution.

Please contact Sales@Pre-Switch.com