HCdP Electronics is based in Bellevue, Pretoria and specialises in the manufacture of power electronics.
The company’s product range includes switch mode power supplies – from 25 W to 10 kW – and battery chargers for lead-acid, NiCad, NiMH and Li-Ion batteries. Its customer base includes such notables as Denel Aerospace, Echo 4x4, Bundu Trailers, Netstar and Healthy Swim, to mention just a few.
Background
Managing director, Manie du Preez’s career and passion as a power electronics design engineer began in the offices of Kentron in 1978. He held the position of technical specialist in the design of missile related power electronics and also served as an advisor on electromagnetic interference related projects. During 1989 his interest in automotive electronics led to the registration of a closed corporation named Dupec Automotive Electronics.
Towards the end of 1989, Delta Motor Corporation took an interest in some of the products of this young company. This led to a joint venture with Delta and the registration of Dupec Electronics. Du Preez resigned from Kentron and was appointed managing director of the new business, whose focus was the design, manufacture and supply of automotive electronic components for the motor industry. Customers included Volkswagen, Delta (now General Motors SA), General Motors Brazil and Renault. Most notable was the development of the Opel Corsa Lite’s fuel injection controller which has been used by Delta from 1998 until now.
In October 1999, Dupec was sold to Electromatic, which later became Shurlok International, where du Preez was duly appointed as technical director. As part of the sale agreement, he entered into a three year contract with Shurlok, followed by a three year restraint of trade in the automotive field.
In 2004, du Preez resigned and started HCdP Electronics.
HCdP Electronics today
HCdP is a young, vibrant business that specialises in power electronics for off-road (4x4), industrial, commercial and military applications.
The company supplies Echo, an OEM trailer manufacturer, with the majority of its off-road electronic components under the Echo brand label. These items include intelligent power panels that display remaining battery capacity and run-time, dual battery switches, battery chargers and solar charge controllers.
In addition to this, HCdP supplies torch manufacturer Lightsaver with specialised quick chargers for its torches. Most recently developed was a 1 A LED driver operating at 1,6 MHz which, together with a new reflector and a Philips Luxeon K2 LED, delivers more than 150 lumen of optical intensity.
A number of 1 kW to 2 kW airborne power supplies have been developed and manufactured for Denel Aerospace. These units have been qualified against strict environmental specifications that include vibration testing, extended temperature operation and performance at 10 000 metre altitude under controlled humidity conditions.
For the past two years, HCdP has also manufactured vehicle tracking cabinets for Netstar. These cabinets contain a number of custom power electronic circuits developed by HCdP, as well as a temperature compensated 25 A, 12 V lead-acid battery charger.
Salt water chlorinator power supplies have also been developed and supplied to Healthy Swim for the past three years. These units keep the rate of chlorine generation constant over a wide range of salt concentrations.
10 kW switch mode power supply
HCdP recently developed a 380 V a.c. to 12 V d.c./110 V d.c. 100 A adjustable power supply for one of its customers. The design consists of a full bridge power stage driving a step-down transformer, followed by a fast diode bridge and an output inductor and filter capacitors. The total development time from order to delivery took approximately 10 weeks. According to HCdP, this was only made possible by committed suppliers like Avnet Kopp and Electrocomp, who supplied key components for the project on time. Some of these components had to be imported as they are not available locally.
The supply’s input power is 12,2 kW when an efficiency of 90% is assumed. Input voltage is 380 V a.c. 3-phase and is rectified by a 1200 V 110 A 6-diode module. Filter capacitors consist of four 4700 μF 350 V capacitors connected in a series-parallel combination with sharing resistors. An inrush current limiter is used to limit the initial charging current to 200 mA. The average DC Link voltage is approximately 532 V d.c.
The full bridge consists of two dual-IGBT modules rated at 75 A and 1200 V. The IGBTs are driven by two driver modules developed by HCdP. Two levels of overload protection are included in the design. The first level of protection consists of a current transformer and associated circuit that will stop the drive to all IGBTs within 2 μs for 100 ms if IGBT and transformer currents exceed 47 A. After shutdown, the unit automatically enters a soft-start mode. The second level of protection is achieved by monitoring the IGBTs’ saturation voltage. If IGBT currents exceed 75 A, all IGBT drives are latched in an OFF state. Resetting is achieved by turning the power OFF and ON.
Four E65/32/27 cores were stacked together for this application. The primary winding consists of 17 turns of six parallel 0,9 mm Grade 3 API wires. The secondary winding is made of five turns of 35 mm x 0,7 mm copper sheet. The diameter of wire and thickness of copper is based on the current penetration dept at the operating frequency.
Initial testing revealed that winding temperatures stabilised at 120°C at 2500 W, a temperature rise of 90°C above ambient. This would have presented serious problems at 10 kW output power. The transformer was subsequently re-modelled, taking Eddy current losses into account and based on work done by P.L. Dowell in 1966.
A revised transformer was designed and tested. Test results showed a maximum temperature rise of only 35°C at 2500 W, a substantial improvement. Total transformer losses at full power were now less than 1% of total output power. According to du Preez, although most literature on transformer losses concentrates on core and copper losses, the importance of Eddy current losses should never be underestimated in high power applications, because they often exceed all other losses.
The peak flux density of the power supply under static operating conditions is 0,17 Tesla, which is well below the saturation limit of 0,35 Tesla for N87 material. Wire, core material and insulation tape were chosen to allow continuous operation at 180°C. The maximum operating temperature at full load is about 100°C, which is optimum to minimise core losses. The total mass of the transformer is 1,25 kg compared to about 135 kg for a conventional 3-phase 50 Hz transformer.
Two ISOTOP ultra-fast recovery diode modules rated at 300 V and 100 A per diode were used as the bridge rectifier. The additional loss of a full bridge rectifier was preferred to a centre-tapped transformer because it is much easier to remove heat from a diode module than it is from a transformer.
The output filter presented a challenge of its own in terms of optimising inductor size, ripple current, full-load and no-load performance. Eventually three T400-26 iron powder cores were stacked and wound with 20 turns of six parallel 2 mm Grade 2 API wires. The overall size is about 120 mm diameter and 70 mm high with no-load inductance of 150 μH, dropping to 45 μH at 100 A due to a drop in incremental permeability. Output capacitors consist of five 1000 μF 200 V low-ESR devices with 100 nf polypropylene capacitors in parallel.
In terms of control circuitry, a pulse with modulated variable frequency control topology was implemented. The variable frequency control adjusts the frequency from 20 kHz to 30 kHz depending on the set output voltage. At low output voltages (12 V) the duty cycle is about 0,11, which would require an ON time of 1,8 μs at 30 kHz and 2,7 μs at 20 kHz.
Since an ON time of 1,8 μs is about the minimum that can be reliably achieved with IGBTs, the variable frequency scheme was implemented to ensure reliable operation at 12 V d.c. output without encountering pulse skipping. The control circuit also ensures a dead time of 3 μs to prevent cross conduction of the IGBTs when operating at maximum duty cycle (110 V d.c.) and minimum input voltage.
The control circuit incorporates a voltage and current control loop. The voltage control loop has two poles and two zeros in order to ensure stability under all operating conditions. The current limit is implemented by sensing the voltage developed over a 1 m sense resistor in series with the output leads.
For more information contact Manie du Preez, HCdP Electronics, +27 (0)12 804 1377, [email protected], www.hcdpelectronics.co.za
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