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At Richardson RFPD, we distribute the power electronic components and sub-systems needed to optimize designs for High-Frequency (HF) Welding Inverters. Our engineers will help you select the right filter capacitors, power factor correction (PFC) capacitors, DC link capacitors, snubber capacitors, current/voltage transducers (sensors), heatsinks, diodes/rectifiers, MOSFETs, IGBTs, power resistors, and gate drivers to best fit your HF welding inverter design requirements.
In addition to being a leading global electronics distributor, our engineers provide design assistance for almost any high-power electronic component used in HF welding systems. Our Energy Management and Power Conversion engineering team has over thirty years experience with inverter designs, in addition to over twenty years experience with both single-phase and 3-phase power supply designs.
|Table of Contents:
Overview of High-Power Electronic HF Welding Systems
In just one of its many uses, high-power electronic HF welding systems employ powerful RF currents at frequencies normally between 100 kHz and 850 kHz to quickly heat formed metals to their melt point (under mechanical pressure), thus performing a very high-quality weld. In this way, HF welding is used to complete the metal forming process for strong and durable metal pipes, tubes, I-Beams and other products too numerous to mention. The detailed physics that drives the process is beyond the scope of this application piece, but at a high level, it can be described this way:
A combination of dense magnetic flux and huge RF currents are generated and applied to the material to be welded as it moves rapidly through the weld head. The RF current (flowing through the material’s “intrinsic resistance”) heats it to just at its melt point, while mechanical pressure is simultaneously applied. The near molten metal “seam” is joined together with a strong molecular bond. The result is a high-speed, high-quality diffusion weld (but commonly referred to as a “forge weld” in the growing HF welding industry).
High-Frequency welding has been in use for over five decades. In the not too distant past, vacuum tubes were used to perform the RF power generation functions. However, the solid state welding inverter has now become the dominant choice for the high frequency pipe and tube making process. Solid state welders are available with power rating ranges from 50 kW to over 2 MW (megawatts). To optimize welding speed and power efficiency, the welding frequency can be tailored from 100 kHz to 850 kHz (for most power ratings). Today’s welders can be configured for the induction welding process, the contact welding process, or both. Load matching is now mostly automatic. Solid state welders are now considered the proven technology for all high frequency pipe or tube welding applications.
The “engine” behind HF welding systems is the high-powered solid state inverter. It produces the large RF currents needed to bring the weld material to its melt point in a very short time. Two main types of inverters are the voltage-fed inverter and the current fed inverter. The main differences between these two circuit topologies are the size and location of the inductors and transformers in the design. Each design has its advantages and disadvantages; and either inverter topology works well in this application.
The main functions of a typical 3-phase HF welding inverter can include input EMI/RFI filtering, AC harmonic filtering, power factor correction, AC-DC conversion, DC Link (i.e., short-term energy storage), a final DC-AC inverter stage, snubbering components, current/voltage transducers (sensors), a high-frequency output transformer, and finally, the actual HF welding head. (Refer to the typical voltage-fed inverter depicted in the block diagram above or via the link below.)
Important specifications to consider when designing HF welding inverters are the required kilowatt (kW) output power level, the “RF weld frequency” (this must be chosen carefully for each welding application and therefore should be somewhat controllable – if possible), the target power efficiency, and the required power factor.
|View the Entire High-Frequency Welding Inverter Block Diagram|
click the image
to view the block diagram
Input Electro-Magnetic Interference/Radio Frequency Interference (EMI/RFI) Filter
Energy from electrical signals (including energy that we do not want) is virtually everywhere: on the power wires coming in from the utility company and promulgating through the air too. UPS circuits have to deal with unwanted energy (e.g., electro-magnetic interference, radio frequency interference, distortion) riding along on the input power lines and filter out the unwanted energy from the desired, relatively clean 3-phase electrical power signals. This is called EMI/RFI filtering. Making sure to reduce and/or eliminate electrical energy from storms, power transients, and other unwanted sources (nearby power generating equipment, high-power radio transmitters, etc.) is the job of the EMI/RFI filter. Also, the high-powered UPS system itself could potentially conduct unwanted energy, generated by high switching currents, back out onto the input power lines. The EMI/RFI filter works to filter-out this unwanted energy as well. Both X2 and Y2 filter-type capacitors are needed.
- X2 Capacitors are specifically designed to be placed from line to line (see block diagram).
- Y2 Capacitors are specifically designed to be placed from line to earth ground (see block diagram).
EMI/RFI Filter Module Solutions
In addition to the discrete component EMI/RFI solution of X2 and Y2 capacitors, we offer a "module" -based solution, consisting of complete, self-contained EMI/RFI filter modules from Spectrum Advanced Specialty Products. Both single-phase and 3-phase "power line filters" are available in many different configurations (e.g., single-stage, dual-stage, delta-connection, wye-connection).
AC Harmonic Filter
HF welding systems definitely apply very high RF power to what can only be characterized as nonlinear load impedances. It is very important to reduce the total harmonic distortion (i.e., current and voltage waveforms at frequencies that are multiples of the fundamental grid frequency) which occurs as a result. These harmonics must be removed, as the welding inverter system must not corrupt either the utility/grid or a generator (if the HF welder happens to be running on back-up generator power) with harmonic currents and voltages. These harmonic (non-sinusoidal) waveforms can cause damage, especially on a long-term basis, to the power utility’s network elements (e.g., transformers), to the welding inverter system itself, or to the back-up generator.
The total harmonic distortion, or THD, of a signal is a measurement of the amount of harmonic distortion present in that signal. It is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
To protect against a high THD measurement, either a passive or an active AC Harmonic Filter is used. Such a filter removes much of the distortion, bringing the THD measurement down. A large, unprotected 3-phase HF welder with a 6-pulse rectifier can create heavy harmonic feedback. It can cause grid voltage distortion or generator malfunction. The Harmonic Filter typically mitigates harmonics to less than 5%. Any power capacitors used in the harmonic filter need to be able to handle relatively large ripple currents at the various harmonic frequencies. Note that the Harmonic Filter also helps with Power Factor Correction – since, as we eliminate harmonic distortion, we bring the circuit closer to the desired unity PF measurement of 1.0 (see the Power Factor and Power Factor Correction, or PFC, section below).
The first step in converting the input 3-phase AC power is rectification. This stage is typically accomplished with six large rectification diodes (a.k.a. "rectifiers"), working together in a bridge-rectifier configuration. Each rectifier needs to be able to withstand the peak voltage and peak current specified for the circuit. When rectifier diodes are used, this is called an "uncontrolled bridge" application.
Alternatively, 6 SCRs (or "Thyristors") can be used in a 3-phase, fully-controlled bridge rectifier circuit. This is generally considered a better design, because the resulting voltage and current waveforms can be controlled so as to be more sinusoidal and, thus, harmonic currents can be reduced.
Whether diodes or SCRs are used, there is typically an "in-rush limiter" (most likely a power resistor) employed at the output of the bridge rectifier. This will help to limit the surge current experienced when power is restored or when the load experiences a transient.
Power Factor and Power Factor Correction (PFC)
The Power Factor (PF) that any AC circuit presents to its source (usually the utility company) is a measure of the ratio of real power (in kW) used by the entire loaded circuit to the total power (a.k.a. "apparent" power), measured as voltage times current (volt-amperes, VA), provided at the input to the circuit.
total power = resistive power + reactive power
kVA (kilovolt-amps) = kW (kilowatts) + kVAR (kilovolt-amps-reactive)
[Mathematically: PF can thus be looked at as simply the magnitude of the cosine and the phase angle between the voltage and current waveforms at the input to the circuit - a number between 0 and 1.]
A utility company (or any power generator system) would like to see a PF as near to 1.0 as possible. In other words, the utility wants to supply the amount of voltage-current product (voltage x current) that is needed by the customer (to perform the work required by the load), and no more. If the PF is low, say 0.8, then the utility needs to supply about 25% more voltage-current product (voltage x current) than it needs to in order to drive the actual load.
If a load is completely resistive, then the current and voltage waveforms are in phase, and the power factor is exactly 1.0. However, most loads are either somewhat inductive (current lags the voltage) or somewhat capacitive (current leads the voltage) and therefore present a "reactive" load to the power source... and the power factor is therefore less than 1.0 in almost all cases.
In practice, many uncorrected loaded circuits have Power Factors that are much lower than unity. Here are a few examples:
|Load Type||Typical Uncorrected|
|Induction Motor||0.70 - 0.90|
|Small Adjustable Speed Drive||0.90 - 0.98|
|0.70 - 0.80|
|0.90 - 0.95|
|Arc Welders||0.35 - 0.80|
Power Factor is so important that utility companies have regulations on how low they will allow a customer's PF. The power company may even impose fines to encourage the customer to provide proper power factor correction (PFC), adding circuitry which will improve the overall power factor until it is closer to unity. The customer's electrical bill itself is normally enough incentive for them to improve their PF. The utility measures both total power delivered (in kilovolt-ampere-hours, kVAH) and real power (in kilowatt-hours, kWH) at the meter. If the PF is below the accepted level, the power company will normally charge the subscriber for the extra power delivered, over that which would be delivered if the acceptable PF had been reached.
Power Factor Correction (PFC) Circuitry
For single-phase circuits, it is relatively easy to provide a PFC circuit that can improve the PF to somewhere between 0.95 and unity. Either passive or active circuits can be employed, but for high-power (>1kW), active PFC circuits are generally employed. A combination of active devices, passive power components (mainly power film capacitors and an inductor), and control circuitry act together to bring the voltage and current waveforms closer to an "in-phase" condition (PF closer to unity).
With 3-phase circuits, a single-phase PFC circuit (passive or active) is generally employed on each of the 3 phases. To achieve very high power factor (0.99), a true 3-phase high frequency active PFC circuit design is currently seen as an emerging technology. These new, high-tech PFC circuits are very compact due to high frequency switching techniques. The reason it is not yet universally accepted (unlike the single phase PFC) is that there are no off-the-shelf controllers available; currently, a DSP driven design is required.
In this part of the circuit, a “power converter” is employed on each phase to control the voltage and current output supplied to the energy storage portion (DC Link) of the HF welding voltage-fed inverter. If a higher voltage level is desired, a Boost Converter (or “step-up” converter) is employed to increase the voltage at the DC Link (compared to the average voltage level at the output of the rectifier/PFC stage). If a lower voltage level is desired, a Buck Converter (or “step-down” converter) is employed to decrease the voltage at the DC Link (compared to the average voltage level at the output of the rectifier/PFC stage). If the DC Link voltage is intended to vary higher or lower than the average level at the rectifier/PFC output, then a Buck-Boost converter is employed. A Buck-Boost converter will adapt to supply lower or higher voltages, depending on the demands of the load.
In any case, the power converter stage uses switching power semiconductors (e.g., IGBTs, MOSFETS), diodes, power passive components (e.g., capacitors, inductors, resisters), and feedback/control circuitry to step-up, step-down, or otherwise regulate the voltage and current outputs which are supplied to the DC Link.
This portion of the HF welder inverter circuit is where the regulated voltage and current waveforms are filtered (i.e. “ripple” is removed) and the energy from all three phases is stored as “DC power.” Note also that the “DC Link voltage” is also called the “bus voltage.”
The DC Link is usually a large capacitor bank made up of either large aluminum electrolytic or power film capacitors, or a combination thereof. These highly specialized capacitors need to be rated to withstand high ripple currents, high temperatures (in some cases), and the high “bus voltage.” They require a high capacitance to volume ratio, high stability (electrical, mechanical, and thermal), low dissipation factor (i.e., low ESR), low ESL (equivalent series inductance), very high insulation resistance, and long working life expectancy.
We provide one of the most diverse DC Link capacitor product assortments in the market today. Because each inverter application is different, and because it is important to choose the right components for each circuit, we supply DC Link capacitors from Cornell Dubilier (CDE), KEMET, Kendeil, Spectrum Advanced Specialty Products, United Chemi-Con, and Vishay.
Final Inverter Stage
The final inverter stage is powered by DC Link power. It is this inverter stage that converts DC energy into the relatively enormous RF signal currents needed to weld. The inverter stage MOSFETs or IGBTs switch at RF welding frequencies (100 kHz – 800 kHz). This stage is generally made-up of very large, high current semiconductor devices (e.g., MOSFETs/IGBTs + diodes and/or thyristor/SCRs).
Richardson RFPD provides one of the most complete offerings of power semiconductor inverter devices in the industry. We offer gate drivers, diodes, IGBTs, MOSFETs, thyristor/SCRs, power IPM transistors, power semiconductor assemblies, and integrated power modules from key suppliers, such as: Dynex, EDI, Microsemi, Mitsubishi, Powerex, Semikron, and Vincotech.
To create the high-power RF voltage and current waveforms needed to weld, the inverter devices are switched on and off at the optimum welding frequency. As a result, voltage and current spikes are developed and these must be carefully removed through a process called “snubbering.” A snubber circuit is typically a carefully chosen capacitor/resistor combination.
The object of the snubber circuit is to eliminate the voltage transient and ringing that occurs when an inverter "switch" opens (i.e., IGBT turns off). The snubber provides an alternate path for the instantaneous current flowing through the inverter device's intrinsic leakage inductance.
Snubber circuits in UPS systems can provide any/all of these three valuable functions:
- Shape the load line of a bipolar switching transistor (IGBT) to help keep it in its safe operating area.
- Remove energy from a switching transistor and dissipate the energy in a resistor to reduce junction temperature.
- Reduce ringing to limit the peak voltage on a switching transistor or rectifying diode and to reduce EMI by reducing emissions and lowering their frequency.
To learn more about designing an RC snubber circuit, please click here.
Current/Voltage Transducers (Sensors)
The current and/or voltage transducers (sensors) make sure that the system is providing the right levels of both current and voltage to the welding load. If a level begins to drop too low, the sensor feeds this information back to the inverter circuit and directs the inverter to try and increase the power to stabilize the current and/or voltage in question. Alternately, if a current and/or voltage level begins to rise too high, the sensor directs the circuitry to produce a lower amount of power. If the circuitry is unable to adjust, then special control signals indicate the need for maintenance on the system.
We offer Hall Effect Current and Voltage Transducers (sensors) from Tamura Corporation.
Thermal Management Systems
Heat Sinks are employed mainly to increase the surface area available for heat transfer from high-power semiconductor devices (e.g., Power Rectifiers, Power Inverter Modules, IGBTs, MOSFETs, Thyristor Modules, Diode Modules), systematically reducing each device's external case temperature, as well as its internal junction temperature. We provide four different categories of heat sinks:
- Snubber Capacitor Application Guide (located on our website in the Design Resource Center)
- Powerex: Using IGBT and IPM Modules (General Considerations - Design Guide)
- More Information about Ultracapacitors
- High-Frequency Welding
Other Related Information and References
- Allowable Harmonic Content (Mains) Standards
- Economics of Power Factor Correction in Large Facilities - Pacific Gas and Electric Co.
- Comparative Evaluation of Control Techniques for a Three-Phase, Three switch Buck-Type AC-to-DC PWM Converter System
- 35 kW Active Rectifier with Integrated Power Modules
- Galvanically Isolated 3 Phase PFC Topologies
Aron Levy: Galvanically Isolated 3 Phase PFC Topologies
General Welding Standards