DIODES AND UNCONTROLLED RECTIFIERS DL DCA201.1 Power Electronics DL DCA201.1 blank page II ver b20120131 DL DCA201.1 CONTENTS 1. THEORETICAL SECTION 1.1. INTRODUCTION Page 1 1.2 RECTIFIER DIODES Page 2 1.2.1 Selenium rectifier cell Page 2 1.2.2 Silicon diode Page 2 1.2.3 Main parameters of diodes Page 2 1.3 UNCONTROLLED RECTIFIERS Page 3 1.4 SINGLE-PULSE RECTIFIER E1UK Page 4 1.4.1 Inductive load Page 5 1.5 TWO-PULSE MIDPOINT RECTIFIER M2UK Page 6 1.6 TWO-PULSE BRIDGE RECTIFIER B2U Page 8 1.7 THREE-PULSE MIDPOINT RECTIFIER M3UK Page 10 1.8 SIX-PULSE MIDPOINT RECTIFIER M6UK Page 12 1.9 SIX-PULSE BRIDGE RECTIFIER B6U Page 14 2. INFORMATIONS 2.1 EXPERIMENT COMPONENTS Page 17 2.2 SETTING UP AND CONDUCTING EXPERIMENTS Page 17 2.3 MEASUREMENTS WITH OSCILLOSCOPE Page 18 2.3.1 No mains isolation Page 18 2.3.2 Power supply via a transformer with isolated windings Page 19 2.3.3 Current measurement Page 20 2.3.4 Current and voltage measurement Page 20 2.3.5 Isolation amplifier Page 21 2.4 SAFETY INFORMATION Page 21 ver b20120131 III DL DCA201.1 EXPERIMENTS EXPERIMENT N°2 Silicon diode Page 29 EXPERIMENT N°3A Single-pulse rectifier E1UK, ohmic load Page 33 EXPERIMENT N°3B Single-pulse rectifier E1UK, ohmic-inductive load Page 39 EXPERIMENT N°4A Three-pulse rectifier M2UK, ohmic load Page 45 EXPERIMENT N°4B Three-pulse rectifier M2UK, ohmic-inductive load Page 51 EXPERIMENT N°5A Two-pulse bridge rectifier B2UK, ohmic load Page 57 EXPERIMENT N°5B Two-pulse bridge rectifier B2UK, ohmic-inductive load Page 63 EXPERIMENT N°6A Three-pulse rectifier M3UK, ohmic load Page 69 EXPERIMENT N°6B Three-pulse rectifier M3UK, ohmic-inductive load Page 77 EXPERIMENT N°7A Six-pulse rectifier M6UK, ohmic load Page 85 EXPERIMENT N°7B Six-pulse rectifier M6UK, ohmic-inductive load Page 93 EXPERIMENT N°8A Six-pulse bridge rectifier B6UK, ohmic load Page 101 EXPERIMENT N°8B Six-pulse bridge rectifier B6UK, ohmic-inductive load Page 107 IV ver b20120131 DL DCA201.1 1. THEORETICAL SECTION 1.1 INTRODUCTION Power electronics performs the application of electronic devices to conversion and control of electrical power with the greatest possible efficiency. The origins of power electronics may be associated to the introduction of the mercury arc power rectifiers, occurred before 1940. To day power electronics has evolved thanks to solid state devices of small dimensions, high efficiency, good reliability and high mechanical strength. These last devices form the base of the most important circuits that perform the control of the power supplied by the mains to the actuator in the automatic control systems. In short these devices are classified as follows: uncontrolled ac/dc converters (rectifiers) controlled ac/dc converters controlled ac/ac converters dc/dc converters (choppers) dc/ac converters (inverters) ver b20120131 1 DL DCA201.1 1.2 RECTIFIER DIODES The most simple device used in uncontrolled ac/dc converters is the rectifier diode which has two terminals, anode A and cathode K. The diode has an asymmetrical volt-ampere characteristic, as shown in the following Fig.1. Fig.1 Diode symbol and volt-ampere characteristic The diode conducts in one direction only and acts as a switch that automatically switches-on when the anode potential is positive in comparison to the cathode potential while automatically switches off when the anode-cathode potential difference is negative and smaller than the breakdown voltage. Commonly used uncontrolled rectifiers include selenium cells and silicon diodes. 1.2.1 Selenium rectifier cell The selenium cell has an aluminium base with several layers of metallic alloys and semiconductive selenium with a polycrystalline structure. The cell is an asymmetrical resistance, low in the forward direction and very high in the reverse direction. The cell is currently used in rectifier circuits at low voltage and designed to withstand surge currents and also used in some circuits for protection against transient overvoltages. 1.2.2 Silicon diode Silicon diode is made of monocrystalline semiconductor chip with P-N junction which permits the easy flow of charge in one direction but restrains the flow in the opposite direction. Silicon diodes replace selenium cells in numerous applications and are used for uncontrolled rectifiers of all power ratings and as free-wheeling diodes. 1.2.3 Main parameters of diodes U(TO) = threshold voltage UF = diode forward voltage at direct current IF IFRM = repetitive peak forward current IFRMS = root-mean-square of the forward current IF IFAV = average forward current, rectified current IFSM = surge forward current, non repetitive URRM = repetitive peak reverse voltage URSM = surge reverse voltage, non repetitive IR = diode reverse current UBR = breakdown voltage trr = reverse recovery time 2 ver b20120131 DL DCA201.1 1.3 UNCONTROLLED RECTIFIERS Static rectifier with diodes performs the basic function of dc rectification: the output voltage of a rectifier is a pulsating unipolar voltage with an average value. The common type of ac/dc converter is composed by a transformer and a diode rectifier circuit, as shown in the following Fig.2. Fig.2 Uncontrolled ac/dc converter In considering rectifier circuits it is important to know the relationships between the input (rms values) and out (average values) parameters, which have the following specifications: Uv = alternating line voltage (r.m.s.) Iv = alternating line current (r.m.s.) Ud = output dc voltage Id = output dc current Afterwards the main rectifier circuits and their identification alphanumeric code are illustrated. ver b20120131 3 DL DCA201.1 1.4 SINGLE PULSE RECTIFIER E1UK The single pulse rectifier, or half wave rectifier illustrated in Fig.3, represents the simplest form of static ac/dc converter. Fig.3 Single pulse rectifier E1UK When an ohmic load R is applied, the diode switches the positive half of the ac voltage Uv to the load and blocks the negative half-wave. As a result, the dc voltage Ud is made up of intermittent sinusoidal half-waves: the direct current Id follows the same sinusoidal time profile as the dc voltage and is in phase with the latter, as illustrated in the following Fig.4. Fig.4 Voltage and current time profiles E1UK 4 ver b20120131 DL DCA201.1 The rectified voltage can be considered as a voltage which varies about the average dc component UdAV. All the following characteristic values apply to resistive load, neglecting losses in the rectifier. 1) Average value of direct voltage UdAV = 0.45 Uv 2) Rms value of direct voltage UdRMS = 0.707 Uv 3) Form factor of direct voltage U f dRMS 1.57 U dAV 4) Rms value of direct current IdfRMS = 1.57 IdAV 5) Ripple factor w = 100 f 2 1 = 121% 6) Rms value of alternating part of direct voltage UdAC = 1.21 UdAV 7) Repetitive peak reverse voltage value of the diode URRM = 3.14 UdAV = 1.41 Uv 1.4.1 Inductive load If we examine the theoretical load case of a purely inductive load, the diode conducts at the positive zero transition of the ac voltage Uv and the current Id increases continuously: the choke stores magnetic energy. At the next zero transition with negative ac voltage, the direct current continues to flow at a decreasing rate until the choke has been discharged: since there are no resistive losses, the discharge time equals the charging time and the current conduction time equals the cycle duration T of the ac voltage. If now we consider the real case of an ohmic-inductive R-L series load, the current conduction time varies between T/2 and T, depending on the magnitude of R and L. ver b20120131 5 DL DCA201.1 1.5 TWO-PULSE MIDPOINT RECTIFIER M2UK Midpoint rectifiers must include a transformer with centre-tapped winding on the secondary side to have two partial and equal ac voltages and two diodes, as shown in the following Fig.5. Fig.5 Two-pulse rectifier M2UK If we refer the voltages Uv1 and Uv2 to the midpoint (neutral) these voltages are in phase opposition and therefore the diode V1 conducts the current when the line voltage Uv1 is positive and the diode V2 conducts the current when the line voltage Uv2 is negative : the dc current is thus made up of consecutive sinusoidal half-waves. The voltage and current time profiles are illustrated in the following Fig.6. Fig.6 Voltage and current time profiles M2UK. 6 ver b20120131 DL DCA201.1 All the following characteristic values apply to resistive load, neglecting losses in the rectifier. 1) Average value of direct voltage UdAV = 0.9 Uv1 = 0.9 Uv2 2) Rms value of direct voltage UdRMS = Uv1 = Uv2 3) Form factor of direct voltage U f dRMS 1.11 U dAV 4) RMS value of direct current IdRMS = 1.11 IdAV 5) Ripple factor w = 100 f 2 1 = 48% 6) Rms value of alternating part of direct voltage UdAC = 0.48 UdAV 7) Repetitive peak reverse voltage value of the diodes URRM = 3.14 UdAV = 2.82 Uv1 = 2.82 Uv2 ver b20120131 7 DL DCA201.1 1.6 TWO-PULSE BRIDGE RECTIFIER B2U The bridge circuit has two branch pairs of series diodes, parallel connected, as indicated in the following Fig.7. Fig.7 Bridge rectifier B2U In the bridge circuit (Graetz bridge) alternatively two diodes (V 2 -V3) are forward biased and two (V4 -V1)reverse biased: the current Id flows through the load R during the two half-periods of alternate voltage Uv. The bridge circuit may be considered as two midpoint circuits connected in series, so the voltage and current profiles are those indicated in Fig.6.