NZ248552A - Electroplating power supply: switchable bridge rectifier with output rectified - Google Patents

Description

2485 ■i ' V \ ^lAU(iiss4 y PATENTS FORM 5 Number 248552 PATENTS ACT 1953 Dated August 31, 1993 COMPLETE SPECIFICATION AN ELECTROPLATING SYSTEM We, INDUSTRIAL RESEARCH LIMITED, a New Zealand company, of Gracefield Road, Lower Hutt, New Zealand, and Canterbury University, a New Zealand University of 100 Ham Road, Christchurch, New Zealand do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement. 1 — FIELD OF THE INVENTION 248552 The present invention comprises a power supply for an electroplating system comprising an electroplating bath.

BACKGROUND ART Electroplating is employed to plate a thin layer of metal onto an object. It is used in various areas of industry to plate for example bright work parts of motor vehicles for cosmetic reasons, to plate component parts of machinery for functional reasons for example to give a part a surface coating of a very hard metal, and in various other areas such as in the manufacture of jewellery or cutlery to plate the jewellery or cutlery with thin layer of metal such as silver.

An electroplating system comprises a bath of electrolyte containing the metal ions to be plated onto the object, and into which bath the object to be plated is placed, generally by suspending it in the bath, and a power supply which provides a low voltage, high current flow through the bath from anode to cathode. The object to be plated acts as the cathode and the current flow causes metal ions to be deposited on the object.

To get a thin, even layer of plated metal on the object it is necessary to control the output voltage and current of the electroplating power supply. In a typical electroplating power supply voltags and current are controlled with a variac. The 24855 electroplating power supply comprises a volt meter and ampmeter and the operator sets the voltage and current to obtain the desired plating thickness. The voltage and current will not necessarily remain constant during plating of an object, but will vary as the number of metal ions in the electroplating bath is reduced as metal ions become deposited on the object being plated, and it is common for the output voltage and current of the power supply to simply be adjusted manually by an experienced operator during the plating process. The object of electro plating is generally to obtain a uniform layer of plated metal over the object to a specified thickness, and obtaining a consistently high quality product is largely dependent upon the experience of the operator of the electroplating bath.

SUMMARY OP INVENTION The present invention provides an improved or at least alternative electroplating power supply particularly suited for small to medium scale industry or electroplating operations.

In broad terms the invention cou^ -rises an electroplating power supply, comprising: a rectifying stage to produce a DC from an AC supply, a bridge comprising switching devices to switch the DC from the rectifier, a microcontroller controlling the bridge switching devices to control the levels of the output voltage and output current and user programmable as to the target output voltage and output current and target plating time, and a transformer stage connected to the output of the switching bridge and a second rectifying stage to produce a low voltage, high current DC output for supply to the electroplating bath.

Preferably the electroplating power supply also comprises a current sensor for insertion into the electroplating bath and mean < in the microcontroller to continuously calculate the density per unit area of current flow in the bath, and the microcontroller is arranged to maintain the density of current flow at a user programmed target level.

Preferably the controller is arranged to optionally produce the DC output of the power supply as low voltage, high current pulses for pulsed plating.

Preferably the switching devices of the switching bridge operated at a frequency in the range 10kHz to 30kHz.

Preferably the transformer comprises two separate transformers the primary windings of which are connected in ?4 fi 5 IJTT, ft <rnr Vt* series and the secondaries of which are connected in parallel to increase current and to force the rectifiers to current share.

DESCRIPTION OF DRAWINGS The invention will be further described with reference to the accompanying drawings, by way of example and without intending to be limiting. In the drawings: Fig. 1 is a schematic circuit for a preferred form of the electroplating power supply of the invention, Fig. 2 shows the control panel of the preferred form controller, by way of example, and Fig. 3 is a flow chart of the setup operations prior to each plating job with the preferred form electroplating system.

DETAILED DESCRIPTION OF PREFERRED FORM Circuit Operation.

Referring to Fig 1 the circuitry of the preferred form system will be described. The AC input to rectifier R1 is mains AC which is typically three phase but may be single phase. Rectifier Rl produces a DC potential across capacitor CI. The bridge which is preferably an H bridge as shown applies the DC potential to the transforming stage T. In the preferred form the transforming stage is made up of two transformers Tl and T2, the 248552 primary windings of which are connected in series as shown, through capacitor C2, and the secondary windings of which are connected in parallel as shown. However, in an alternative arrangement the transforming stage T may comprise a single physical transformer with a single primary equivalent to the two primary windings shown, and a two part secondary with the two part secondary windings arranged in parallel, and it is preferred but not essential that the secondary winding comprises two parts arranged in parallel, rather than essential.

The H bridge comprises switching devices SI to S4, one in each ox: its arms. The switching devices are in turn controlled by the controller shown which provides a switching waveform to each of the switching devices Si to S4. The bridge applies the DC potential from the rectifier R1 to the combined primary of the transformer T such that current flows in one direction and then reverses to flow in the opposite direction, which is accomplished by closing switches SI and S4, or S2 and S3. The switching devices SI to S4 may be any suitable form of switching device known but are most preferably insulated gate bi polar transistors but could be any other suitable form of switching device.

The outputs of the parallel secondary windings of transformer T are rectified by rectifiers R2 and R2A and paralleled, to provide a low voltage high current DC output from the power supply to the electroplating bath. 248552 The controller by controlling the switching frequency of the E bridge and the time for which the switching devices SI and S3 and S2 and S4 remain closed/open, controls the output voltage and output current of the electroplating supply. Changing the duty cycle of the switching bridge changes the amount of energy transferred to the output of the transformer stage T. As output current flow decreases, the duty cycle is increased to maintain the current flow at the target current flow.

The controller is user programmable as described further below, so that an operator may program the target output voltage and output current to be supplied by the electroplating supply. For example the target data may be entered into the controller through a small control pad on the front face of the controller casing. The output voltage and current flow from the DC output of the electroplating supply from rectifiers R2 and R2A is input to the controller as shown, to provide a feed back loop for output voltage and current control.

The controller is also user programmable as to the plating time i.e. an operator may key into the controller a target plating time and once plating has been carried out for that time the switching bridge is caused to go open circuit to stop current flow to the electroplating bath. 248552 The electroplating supply may also incorporate a current density probe into the electroplating bath, of known area. The microcontroller is arranged to continuously calculate and monitor the current density, knowing the area of the probe in the electrolyte and the current flow from the probe. The microcontroller adjusts the current flow to maintain a set current density, with the target current density being user programmed into the controller at the start of the plating operation.

Preferably the controller is arranged to switch the switching devices SI to S4 at a frequency in the range 10kHz to 30kHz and most preferably about 20 kHz. This is a relatively high frequency for such a power supply, but allows faster control response of the output voltage and current of the controller, and has also been found to produce lower ripple on the output of the electroplating supply.

To produce a pulsed plating output the controller rapidly changes the duty cycle of the H bridge, so that pulses of plating current are delivered to the electroplating bath. The frequency of the current pulses and their current (no current ratio may be controlled).

User Operation.

Referring to Fig 2, there are five buttons on the control panel of the preferred form electroplahing supply. These are: POWER - pressing this button turns the system on/off« When the power is on, a lamp behind the cover of the power button is illuminated and the LCD will display a message. To switch off, press the button again.

SELECT - the select button is used for moving through the menu system and selecting options.

UP and DOWN - these buttons are used to increase and decrease control parameters such as voltage, current, time and amp-hours. They also toggle between options such as voltage and current control.

START/STOP - this button is used to start and stop the plating process. If the power supply is working, a lamp behind this button is illuminated to indicate that a plating run is in process.

Referring to the flow chart of Fig 3 for the preferred form system, when the controller is first turned on it will be in the Voltage Control mode, the operator can change to Current Control by pressing the UP button. To change back, press the DOWN button. Once either voltage or current control has been selected, press the SELECT button to get to the next part of the menu. 248 5 This part of the menu sets the appropriate control level; either voltage or current. To increase this control value press the UP button and to decrease it press the DOWN button. Once the correct value has been set, pressing the SELECT button again moves down the menu tree.

The appropriate limiting control value is now set; either maximum current or maximum voltage. As before, to change the value use the UP and DOWN buttons and press SELECT when finished.

The process termination mode is now selected. Initially the controller will Stop After a Set Time as displayed on the LCD. To stop after a set number of amp hours the operator presses the UP button. The LCD will now display Stop After Set Amp Hours. To change back to stopping after a set time press the DOWN button. Press SELECT when finished.

The final menu entry allows the operator to set the number of minutes or number of amp hours to stop after depending on the termination mode set in the previous menu. To set the value use the UP and DOWN button as before. When finished press the SELECT button. This returns the display back to the top of the menu, ie selecting between voltage and current control.

The electro-plating job is now started by pressing the START/STOP button. 248552 The electroplating supply of the invention is user programmable as to output current and voltage and plating time and preferably also current density. It enables close control of the rate of deposit of metal on the object being plated, to enable consistent quality to be maintained. Preferably the electroplating supply has the facility for pulsed plating to provide a better control over the thickness of metal deposited on complex shapes. The electroplating supply of the invention can be formed so as to be more compact than larger bulkier supplies which are common.

The foregoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof.

Claims (9)

WHAT WE CLAIM IS: 248552

1. An electroplating power supply, comprising: a rectifying stage to produce a DC from an AC supply, a bridge comprising switching devices to switch the DC from the rectifier, a microcontroller controlling the bridge switching devices to control the levels of the output voltage and output current and user programmable as to the target output voltage and output current and target plating time, and a transformer stage connected to the output of the switching bridge and a second rectifying stage to produce a low voltage, high current DC output for supply to an electroplating bath.

2. An electroplating power supply as claimed in claim 1 also comprising a current sensor for insertion into the electroplating bath and wherein the microcontroller is arranged to continuously calculate the density per unit area of current flow in the bath and to maintain the density of current flow at a user programmed target level. 248552

3. An electroplating power supply as claimed in either one of claims 1 and 2, wherein the microcontroller is arranged to optionally produce the DC output of the power supply as low voltage, high current pulses for pulsed plating.

4. An electroplating power supply as claimed in any one of claims 1 to 3, wherein the switching devices of the switching bridge operate at a frequency in the range 10kHz to 30kHz.

5. An electroplating power supply as claimed in any one of claims 1 to 4, wherein the transformer stage comprises two separate transformers the primary windings of which are connected in series and the secondaries of which are connected in parallel.

6. An electroplating power supply as claimed in any one of the preceding claims wherein the switching devices are insulated gate bi-polar transistors.

7. An electroplating power supply as claimed in any one of the preceding claims, wherein the energy transferred to the transformer stage by the switching bridge is controlled by controlling the duty cycle of the switching bridge.

8. An electroplating power supply substantially as herein described with reference to Fig. 1 of the accompanying drawings. - 13 - 248552

9. An electroplating power supply substantially as herein described with reference to Figs 1 to 3 of the accompanying drawings. WEST-WALKER McCABE ATTORNEYS FOR THE APPLICANT - 14 -